drm/i915: Use our singlethreaded wq for freeing objects

As freeing the objects require serialisation on struct_mutex, we should
prefer to use our singlethreaded driver wq that is dedicated to work
requiring struct_mutex (hence serialised).The benefit should be less
clutter on the system wq, allowing it to make progress even when the
driver/struct_mutex is heavily contended.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180115122846.15193-1-chris@chris-wilson.co.uk
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

drivers/gpu/drm/i915/i915_gem.c

@@ -4732,7 +4732,7 @@ static void __i915_gem_free_object_rcu(struct rcu_head *head)
 	 * detour through a worker.
 	 */
 	if (llist_add(&obj->freed, &i915->mm.free_list))
-		schedule_work(&i915->mm.free_work);
+		queue_work(i915->wq, &i915->mm.free_work);
 }
 
 void i915_gem_free_object(struct drm_gem_object *gem_obj)