drm/i915/selftests: Workaround an issue with unused lockdep subclass

drm/i915/selftests: Workaround an issue with unused lockdep subclass

lockdep insists that if we give a lock a subclass, it must be used.
Failure to do so triggers a self-consistency check when reading
lockdep_stats:

[   49.902002] DEBUG_LOCKS_WARN_ON(debug_atomic_read(nr_unused_locks) != nr_unused)
[   49.902009] WARNING: CPU: 3 PID: 383 at kernel/locking/lockdep_proc.c:249 lockdep_stats_show+0x984/0xa10
[   49.902026] Modules linked in: nls_ascii nls_cp437 vfat fat crct10dif_pclmul crc32_pclmul crc32c_intel aesni_intel aes_x86_64 crypto_simd cryptd glue_helper intel_cstate intel_uncore intel_rapl_perf intel_gtt efivars prime_numbers ahci libahci i2c_i801 video button efivarfs [last unloaded: drm_kms_helper]
[   49.902059] CPU: 3 PID: 383 Comm: cat Tainted: G     U            4.20.0-rc2+ #304
[   49.902068] Hardware name: Intel Corporation NUC7i5BNK/NUC7i5BNB, BIOS BNKBL357.86A.0052.2017.0918.1346 09/18/2017
[   49.902079] RIP: 0010:lockdep_stats_show+0x984/0xa10
[   49.902086] Code: 00 85 c0 0f 84 aa f8 ff ff 8b 05 77 37 e2 00 85 c0 0f 85 9c f8 ff ff 48 c7 c6 e0 57 bc 81 48 c7 c7 28 30 bb 81 e8 6b 77 fa ff <0f> 0b e9 82 f8 ff ff 48 c7 44 24 50 00 00 00 00 45 31 e4 31 db 31
[   49.902103] RSP: 0018:ffffc90000247d58 EFLAGS: 00010292
[   49.902110] RAX: 0000000000000044 RBX: 00000000000002f0 RCX: 0000000000000000
[   49.902118] RDX: 0000000000000002 RSI: 0000000000000001 RDI: ffffffff810b3464
[   49.902126] RBP: 0000000000000039 R08: 0000000000000002 R09: 0000000000000000
[   49.902133] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000007ead
[   49.902141] R13: 0000000000000001 R14: ffff88884c021000 R15: 0000000000000097
[   49.902150] FS:  00007fb347e66540(0000) GS:ffff88885e600000(0000) knlGS:0000000000000000
[   49.902159] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   49.902165] CR2: 00007fb347aeb000 CR3: 00000008544bd005 CR4: 00000000001606e0

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Michał Winiarski <michal.winiarski@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181115203851.25739-1-chris@chris-wilson.co.uk