drm/i915/selftests: Create mock_engine() under struct_mutex

Calling mock_engine() calls i915_timeline_init() and that requires
struct_mutex to be held as it adds itself to the global list of
timelines. This error was introduced by commit a89d1f921c15 ("drm/i915:
Split i915_gem_timeline into individual timelines") but the issue was
masked in CI by the earlier lockdep spam.

Fixes: a89d1f921c15 ("drm/i915: Split i915_gem_timeline into individual timelines")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180508211056.17151-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/selftests/mock_gem_device.c

@@ -229,18 +229,20 @@ struct drm_i915_private *mock_gem_device(void)
 	INIT_LIST_HEAD(&i915->gt.closed_vma);
 
 	mutex_lock(&i915->drm.struct_mutex);
+
 	mock_init_ggtt(i915);
-	mutex_unlock(&i915->drm.struct_mutex);
 
 	mkwrite_device_info(i915)->ring_mask = BIT(0);
 	i915->engine[RCS] = mock_engine(i915, "mock", RCS);
 	if (!i915->engine[RCS])
-		goto err_priorities;
+		goto err_unlock;
 
 	i915->kernel_context = mock_context(i915, NULL);
 	if (!i915->kernel_context)
 		goto err_engine;
 
+	mutex_unlock(&i915->drm.struct_mutex);
+
 	WARN_ON(i915_gemfs_init(i915));
 
 	return i915;
@@ -248,7 +250,8 @@ struct drm_i915_private *mock_gem_device(void)
 err_engine:
 	for_each_engine(engine, i915, id)
 		mock_engine_free(engine);
-err_priorities:
+err_unlock:
+	mutex_unlock(&i915->drm.struct_mutex);
 	kmem_cache_destroy(i915->priorities);
 err_dependencies:
 	kmem_cache_destroy(i915->dependencies);