drm/i915/selftests: Initialise mock_i915->mm.obj_lock

drm/i915/selftests: Initialise mock_i915->mm.obj_lock

lockdep spotted that the mock tests were using the i915->mm.obj_lock
without first initialiasing it:

>[ 1303.217043] [IGT] drv_selftest: starting subtest mock_objects
<4>[ 1303.240898] Setting dangerous option mock_selftests - tainting kernel
<6>[ 1303.253665] i915: Performing mock selftests with st_random_seed=0xd87ea6c6 st_timeout=1000
<4>[ 1303.254812] INFO: trying to register non-static key.
<4>[ 1303.254816] the code is fine but needs lockdep annotation.
<4>[ 1303.254818] turning off the locking correctness validator.
<4>[ 1303.254820] CPU: 4 PID: 13112 Comm: drv_selftest Tainted: G     U  W       4.14.0-rc8-CI-Patchwork_7058+ #1
<4>[ 1303.254823] Hardware name: MSI MS-7924/Z97M-G43(MS-7924), BIOS V1.12 02/15/2016
<4>[ 1303.254825] Call Trace:
<4>[ 1303.254829]  dump_stack+0x68/0x9f
<4>[ 1303.254832]  register_lock_class+0x3fd/0x580
<4>[ 1303.254835]  ? ___slab_alloc.constprop.29+0x157/0x3d0
<4>[ 1303.254837]  ? ___slab_alloc.constprop.29+0x157/0x3d0
<4>[ 1303.254840]  ? sg_kmalloc+0x1e/0x50
<4>[ 1303.254842]  ? debug_smp_processor_id+0x17/0x20
<4>[ 1303.254845]  __lock_acquire+0xa4/0x1b00
<4>[ 1303.254884]  ? __i915_gem_object_set_pages+0x116/0x1f0 [i915]
<4>[ 1303.254887]  ? __this_cpu_preempt_check+0x13/0x20
<4>[ 1303.254889]  ? sg_kmalloc+0x1e/0x50
<4>[ 1303.254891]  lock_acquire+0xb0/0x200
<4>[ 1303.254893]  ? lock_acquire+0xb0/0x200
<4>[ 1303.254917]  ? __i915_gem_object_set_pages+0x116/0x1f0 [i915]
<4>[ 1303.254920]  _raw_spin_lock+0x32/0x50
<4>[ 1303.254944]  ? __i915_gem_object_set_pages+0x116/0x1f0 [i915]
<4>[ 1303.254967]  __i915_gem_object_set_pages+0x116/0x1f0 [i915]
<4>[ 1303.254991]  i915_gem_object_get_pages_phys+0x286/0x2b0 [i915]
<4>[ 1303.255015]  ____i915_gem_object_get_pages+0x20/0x60 [i915]
<4>[ 1303.255039]  i915_gem_object_attach_phys+0x137/0x1a0 [i915]
<4>[ 1303.255063]  igt_phys_object+0x45/0x120 [i915]
<4>[ 1303.255094]  __i915_subtests+0x40/0xd0 [i915]
<4>[ 1303.255099]  ? work_on_cpu_safe+0x60/0x60
<4>[ 1303.255131]  i915_gem_object_mock_selftests+0x34/0x50 [i915]

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171110151919.18451-1-chris@chris-wilson.co.uk
Reviewed-by: Matthew Auld <matthew.william.auld@gmail.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>