drm/i915/icl: Fix crash when getting DPLL of a MST encoder in TC ports

drm/i915/icl: Fix crash when getting DPLL of a MST encoder in TC ports

enc_to_dig_port() returns NULL for encoders of type
INTEL_OUTPUT_DP_MST causing the crash bellow:

[ 2832.836101] BUG: unable to handle kernel paging request at 00000000000012b8
[ 2832.843062] PGD 0 P4D 0
[ 2832.845610] Oops: 0000 [#1] SMP
[ 2832.848764] CPU: 2 PID: 3577 Comm: kworker/2:0 Tainted: G        W         4.19.0-rc7+ #491
[ 2832.857106] Hardware name: Intel Corporation Ice Lake Client Platform/IceLake U DDR4 SODIMM PD RVP TLC, BIOS ICLSFWR1.R00.2352.A01.1808281852 08/28/2018
[ 2832.870734] Workqueue: events output_poll_execute
[ 2832.875480] RIP: 0010:icl_get_dpll+0xa4/0x5d0 [i915]
[ 2832.880449] Code: e9 03 f3 48 ab 8b 6e 74 41 8b 8c 24 5c 03 00 00 85 ed 0f 88 3f 02 00 00 83 fd 01 0f 8e ad 01 00 00 83 fd 05 0f 8f 2d 02 00 00 <83> ba b8 12 00 00 02 48 8b 36 0f 84 39 02 00 00 44 8b be ec 89 00
[ 2832.899176] RSP: 0018:ffffc90001b57a78 EFLAGS: 00010293
[ 2832.904404] RAX: 0000000000000000 RBX: ffffc90001b57a94 RCX: 0000000000083d60
[ 2832.911536] RDX: 0000000000000000 RSI: ffff8804a8c0dc00 RDI: ffffc90001b57b18
[ 2832.918668] RBP: 0000000000000003 R08: ffff8804a8c1f990 R09: ffff8804a8c1f990
[ 2832.925797] R10: 0000000000000000 R11: ffff8804a8e99600 R12: ffff8804a7760000
[ 2832.932930] R13: ffff88049e94d000 R14: ffff88049e94d000 R15: 000000000000000e
[ 2832.940063] FS:  0000000000000000(0000) GS:ffff8804b0300000(0000) knlGS:0000000000000000
[ 2832.948147] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 2832.953893] CR2: 00000000000012b8 CR3: 0000000004a1d004 CR4: 0000000000760ee0
[ 2832.961027] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 2832.968155] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[ 2832.975286] PKRU: 55555554
[ 2832.978003] Call Trace:
[ 2832.980496]  haswell_crtc_compute_clock+0x3d/0x68 [i915]
[ 2832.985841]  intel_crtc_atomic_check+0x61/0x340 [i915]
[ 2832.990987]  drm_atomic_helper_check_planes+0x130/0x1c0
[ 2832.996245]  intel_atomic_check+0x4d5/0x10f0 [i915]
[ 2833.001147]  drm_atomic_check_only+0x484/0x690
[ 2833.005629]  drm_atomic_commit+0x13/0x50
[ 2833.009564]  restore_fbdev_mode_atomic+0x1c9/0x1e0
[ 2833.014363]  drm_fb_helper_restore_fbdev_mode_unlocked+0x47/0x90
[ 2833.020368]  drm_fb_helper_set_par+0x29/0x50
[ 2833.024641]  drm_fb_helper_hotplug_event.part.33+0x92/0xb0
[ 2833.030130]  drm_kms_helper_hotplug_event+0x26/0x30
[ 2833.035013]  output_poll_execute+0x192/0x1b0
[ 2833.039293]  process_one_work+0x2a5/0x5f0
[ 2833.043315]  worker_thread+0x2d/0x3d0
[ 2833.046988]  ? rescuer_thread+0x340/0x340
[ 2833.051009]  kthread+0x112/0x130
[ 2833.054247]  ? kthread_create_worker_on_cpu+0x70/0x70
[ 2833.059307]  ret_from_fork+0x3a/0x50
[ 2833.062893] Modules linked in: i915 prime_numbers snd_hda_codec_realtek snd_hda_codec_generic asix snd_usb_audio snd_usbmidi_lib snd_seq_midi snd_seq_midi_event snd_rawmidi cdc_ether usbnet x86_pkg_temp_thermal xhci_pci xhci_hcd ucsi_acpi typec_ucsi typec efivarfs [last unloaded: prime_numbers]
[ 2833.088917] CR2: 00000000000012b8
[ 2833.092241] ---[ end trace 25f9fe3d47af2e75 ]---
[ 2833.096895] RIP: 0010:icl_get_dpll+0xa4/0x5d0 [i915]
[ 2833.101866] Code: e9 03 f3 48 ab 8b 6e 74 41 8b 8c 24 5c 03 00 00 85 ed 0f 88 3f 02 00 00 83 fd 01 0f 8e ad 01 00 00 83 fd 05 0f 8f 2d 02 00 00 <83> ba b8 12 00 00 02 48 8b 36 0f 84 39 02 00 00 44 8b be ec 89 00
[ 2833.120589] RSP: 0018:ffffc90001b57a78 EFLAGS: 00010293
[ 2833.125815] RAX: 0000000000000000 RBX: ffffc90001b57a94 RCX: 0000000000083d60
[ 2833.132946] RDX: 0000000000000000 RSI: ffff8804a8c0dc00 RDI: ffffc90001b57b18
[ 2833.140080] RBP: 0000000000000003 R08: ffff8804a8c1f990 R09: ffff8804a8c1f990
[ 2833.147213] R10: 0000000000000000 R11: ffff8804a8e99600 R12: ffff8804a7760000
[ 2833.154350] R13: ffff88049e94d000 R14: ffff88049e94d000 R15: 000000000000000e
[ 2833.161483] FS:  0000000000000000(0000) GS:ffff8804b0300000(0000) knlGS:0000000000000000
[ 2833.169565] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 2833.175313] CR2: 00000000000012b8 CR3: 0000000004a1d004 CR4: 0000000000760ee0
[ 2833.182449] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[ 2833.189578] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[ 2833.196712] PKRU: 55555554

MST ports are allocated from struct intel_dp_mst_encoder not from
struct intel_digital_port as regular ports, so to get the TC type it
is necessary check the primary digital port of the mst encoder.

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181030215750.28213-5-jose.souza@intel.com