drm/cirrus: Remove unnecessary NULL check in bo_unref

ttm_bo_unref unconditionally calls kref_put on it's argument, so the
thing can't be NULL without already causing Oopses.

Spotted by coverity.
Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/cirrus/cirrus_main.c

@@ -264,9 +264,7 @@ static void cirrus_bo_unref(struct cirrus_bo **bo)
 
 	tbo = &((*bo)->bo);
 	ttm_bo_unref(&tbo);
-	if (tbo == NULL)
 	*bo = NULL;
-
 }
 
 void cirrus_gem_free_object(struct drm_gem_object *obj)