drm/fb-helper: only unmap if buffer not null

drm/fb-helper: only unmap if buffer not null

drm_fbdev_cleanup() can be called when fb_helper->buffer is null, hence
fb_helper->buffer should be checked before calling
drm_client_buffer_vunmap(). This buffer is also checked in
drm_client_framebuffer_delete(), so we should also do the same thing for
drm_client_buffer_vunmap().

[  199.128742] RIP: 0010:drm_client_buffer_vunmap+0xd/0x20
[  199.129031] Code: 43 18 48 8b 53 20 49 89 45 00 49 89 55 08 5b 44 89 e0 41 5c 41 5d 41 5e 5d
c3 0f 1f 00 53 48 89 fb 48 8d 7f 10 e8 73 7d a1 ff <48> 8b 7b 10 48 8d 73 18 5b e9 75 53 fc ff 0
f 1f 44 00 00 48 b8 00
[  199.130041] RSP: 0018:ffff888103f3fc88 EFLAGS: 00010282
[  199.130329] RAX: 0000000000000001 RBX: 0000000000000000 RCX: ffffffff8214d46d
[  199.130733] RDX: 1ffffffff079c6b9 RSI: 0000000000000246 RDI: ffffffff83ce35c8
[  199.131119] RBP: ffff888103d25458 R08: 0000000000000001 R09: fffffbfff0791761
[  199.131505] R10: ffffffff83c8bb07 R11: fffffbfff0791760 R12: 0000000000000000
[  199.131891] R13: ffff888103d25468 R14: ffff888103d25418 R15: ffff888103f18120
[  199.132277] FS:  00007f36fdcbb6a0(0000) GS:ffff88815b400000(0000) knlGS:0000000000000000
[  199.132721] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  199.133033] CR2: 0000000000000010 CR3: 0000000103d26000 CR4: 00000000000006f0
[  199.133420] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  199.133807] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[  199.134195] Call Trace:
[  199.134333]  drm_fbdev_cleanup+0x179/0x1a0
[  199.134562]  drm_fbdev_client_unregister+0x2b/0x40
[  199.134828]  drm_client_dev_unregister+0xa8/0x180
[  199.135088]  drm_dev_unregister+0x61/0x110
[  199.135315]  mgag200_pci_remove+0x38/0x52 [mgag200]
[  199.135586]  pci_device_remove+0x62/0xe0
[  199.135806]  device_release_driver_internal+0x148/0x270
[  199.136094]  driver_detach+0x76/0xe0
[  199.136294]  bus_remove_driver+0x7e/0x100
[  199.136521]  pci_unregister_driver+0x28/0xf0
[  199.136759]  __x64_sys_delete_module+0x268/0x300
[  199.137016]  ? __ia32_sys_delete_module+0x300/0x300
[  199.137285]  ? call_rcu+0x3e4/0x580
[  199.137481]  ? fpregs_assert_state_consistent+0x4d/0x60
[  199.137767]  ? exit_to_user_mode_prepare+0x2f/0x130
[  199.138037]  do_syscall_64+0x33/0x40
[  199.138237]  entry_SYSCALL_64_after_hwframe+0x44/0xae
[  199.138517] RIP: 0033:0x7f36fdc3dcf7

Signed-off-by: Tong Zhang <ztong0001@gmail.com>
Fixes: 763aea17bf57 ("drm/fb-helper: Unmap client buffer during shutdown")
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Cc: <stable@vger.kernel.org> # v5.11+
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210228044625.171151-1-ztong0001@gmail.com
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>