drm/i915: Fix a potential UAF at device unload

i915_gem_drain_freed_objects() might not be enough to
free all the objects and RCU delayed work might get
scheduled after the i915 device struct gets freed.

Call i915_gem_drain_workqueue() to catch all RCU delayed work.
Suggested-by: Chris Wilson <chris.p.wilson@intel.com>
Acked-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220923073515.23093-1-nirmoy.das@intel.com

drivers/gpu/drm/i915/i915_gem.c

@@ -1242,7 +1242,7 @@ void i915_gem_init_early(struct drm_i915_private *dev_priv)
 
 void i915_gem_cleanup_early(struct drm_i915_private *dev_priv)
 {
-	i915_gem_drain_freed_objects(dev_priv);
+	i915_gem_drain_workqueue(dev_priv);
 	GEM_BUG_ON(!llist_empty(&dev_priv->mm.free_list));
 	GEM_BUG_ON(atomic_read(&dev_priv->mm.free_count));
 	drm_WARN_ON(&dev_priv->drm, dev_priv->mm.shrink_count);