drm/i915: Add a reminder that i915_vma_move_to_active() requires struct_mutex

i915_vma_move_to_active() requires the struct_mutex for serialisation
with retirement, so mark it up with lockdep_assert_held().

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161218153724.8439-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem_execbuffer.c

@@ -1259,6 +1259,7 @@ void i915_vma_move_to_active(struct i915_vma *vma,
 	struct drm_i915_gem_object *obj = vma->obj;
 	const unsigned int idx = req->engine->id;
 
+	lockdep_assert_held(&req->i915->drm.struct_mutex);
 	GEM_BUG_ON(!drm_mm_node_allocated(&vma->node));
 
 	/* Add a reference if we're newly entering the active list.