drm/gem: Be more friendly with locking checks

BUG_ON kills the driver, WARN_ON is much friendlier. And usually nothing
bad happens when the locking is slightly busted.

v2: Fix typos in commit message Thierry spotted.

Reviewed-by: Thierry Reding <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>

drivers/gpu/drm/drm_gem.c

···
 	struct drm_gem_object *obj = (struct drm_gem_object *) kref;
 	struct drm_device *dev = obj->dev;
 
-	BUG_ON(!mutex_is_locked(&dev->struct_mutex));
+	WARN_ON(!mutex_is_locked(&dev->struct_mutex));
 
 	if (dev->driver->gem_free_object != NULL)
 		dev->driver->gem_free_object(obj);