drm: Fixup locking WARN_ON mistake around gem_object_free_unlocked

Embarrassingly while fixing up the old paths for i915 I managed to
misplace a locking check for the new _unlocked paths. That's what I
get for not retesting on radeon.

Fixes: 9f0ba539d13a ("drm/gem: support BO freeing without dev->struct_mutex")
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Alex Deucher <alexdeucher@gmail.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Tested-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

drivers/gpu/drm/drm_gem.c

@@ -804,12 +804,13 @@ drm_gem_object_free(struct kref *kref)
 		container_of(kref, struct drm_gem_object, refcount);
 	struct drm_device *dev = obj->dev;
 
-	WARN_ON(!mutex_is_locked(&dev->struct_mutex));
-
-	if (dev->driver->gem_free_object_unlocked)
+	if (dev->driver->gem_free_object_unlocked) {
 		dev->driver->gem_free_object_unlocked(obj);
-	else if (dev->driver->gem_free_object)
+	} else if (dev->driver->gem_free_object) {
+		WARN_ON(!mutex_is_locked(&dev->struct_mutex));
+
 		dev->driver->gem_free_object(obj);
+	}
 }
 EXPORT_SYMBOL(drm_gem_object_free);