drm/i915: Drain the ttm delayed workqueue too

Lets be thorough here. Users of the TTM backend would likely expect this
behaviour.
Signed-off-by: NMaarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: NMatthew Auld <matthew.auld@intel.com>
Signed-off-by: NMatthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211117142024.1043017-6-matthew.auld@intel.com

drivers/gpu/drm/i915/i915_drv.h

@@ -1822,6 +1822,7 @@ static inline void i915_gem_drain_freed_objects(struct drm_i915_private *i915)
 	 */
 	while (atomic_read(&i915->mm.free_count)) {
 		flush_work(&i915->mm.free_work);
+		flush_delayed_work(&i915->bdev.wq);
 		rcu_barrier();
 	}
 }