drm/ast: 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: NDavid Herrmann <dh.herrmann@gmail.com>
Reviewed-by: NIan Romanick <ian.d.romanick@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/ast/ast_main.c

@@ -411,10 +411,9 @@ static void ast_bo_unref(struct ast_bo **bo)
 
 	tbo = &((*bo)->bo);
 	ttm_bo_unref(&tbo);
-	if (tbo == NULL)
-		*bo = NULL;
-
+	*bo = NULL;
 }
+
 void ast_gem_free_object(struct drm_gem_object *obj)
 {
 	struct ast_bo *ast_bo = gem_to_ast_bo(obj);