drm/i915/selftests: Yet another forgotten mock_i915->mm initialiser

drm/i915/selftests: Yet another forgotten mock_i915->mm initialiser

Move all of the i915->mm initialisation to a private function that can
be reused by the mock i915 device to save forgetting any more steps.

For example,
<7>[ 1542.046332] [IGT] drv_selftest: starting subtest mock_objects
<4>[ 1542.123924] Setting dangerous option mock_selftests - tainting kernel
<6>[ 1542.167941] i915: Performing mock selftests with st_random_seed=0x246f5ab5 st_timeout=1000
<4>[ 1542.178012] INFO: trying to register non-static key.
<4>[ 1542.178027] the code is fine but needs lockdep annotation.
<4>[ 1542.178032] turning off the locking correctness validator.
<4>[ 1542.178041] CPU: 3 PID: 6008 Comm: kworker/3:7 Tainted: G     U          4.14.0-rc8-CI-CI_DRM_3332+ #1
<4>[ 1542.178049] Hardware name:                  /NUC6CAYB, BIOS AYAPLCEL.86A.0040.2017.0619.1722 06/19/2017
<4>[ 1542.178144] Workqueue: events __i915_gem_free_work [i915]
<4>[ 1542.178152] Call Trace:
<4>[ 1542.178163]  dump_stack+0x68/0x9f
<4>[ 1542.178170]  register_lock_class+0x3fd/0x580
<4>[ 1542.178177]  ? unwind_next_frame+0x14/0x20
<4>[ 1542.178184]  ? __save_stack_trace+0x73/0xd0
<4>[ 1542.178191]  __lock_acquire+0xa4/0x1b00
<4>[ 1542.178254]  ? __i915_gem_free_work+0x28/0xa0 [i915]
<4>[ 1542.178261]  ? __lock_acquire+0x4ab/0x1b00
<4>[ 1542.178268]  lock_acquire+0xb0/0x200
<4>[ 1542.178273]  ? lock_acquire+0xb0/0x200
<4>[ 1542.178336]  ? __i915_gem_free_work+0x28/0xa0 [i915]
<4>[ 1542.178344]  _raw_spin_lock+0x32/0x50
<4>[ 1542.178405]  ? __i915_gem_free_work+0x28/0xa0 [i915]
<4>[ 1542.178468]  __i915_gem_free_work+0x28/0xa0 [i915]
<4>[ 1542.178476]  process_one_work+0x221/0x650
<4>[ 1542.178483]  worker_thread+0x4e/0x3c0
<4>[ 1542.178489]  kthread+0x114/0x150
<4>[ 1542.178494]  ? process_one_work+0x650/0x650
<4>[ 1542.178499]  ? kthread_create_on_node+0x40/0x40
<4>[ 1542.178506]  ret_from_fork+0x27/0x40

v2: Fish out i915->mm.object_stat_lock which was being inited over in
i915_drv.c (Matthew)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171110232447.21618-1-chris@chris-wilson.co.uk
Reviewed-by: Matthew Auld <matthew.auld@intel.com>