st/mesa: use _mesa_delete_buffer_object

This is more future-proof than the current code.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Cc: "11.0 11.1" <mesa-stable@lists.freedesktop.org>
(cherry picked from commit 1c2187b1c2)

src/mesa/state_tracker/st_cb_bufferobjects.c

@@ -83,9 +83,7 @@ st_bufferobj_free(struct gl_context *ctx, struct gl_buffer_object *obj)
    if (st_obj->buffer)
       pipe_resource_reference(&st_obj->buffer, NULL);
 
-   mtx_destroy(&st_obj->Base.Mutex);
-   free(st_obj->Base.Label);
-   free(st_obj);
+   _mesa_delete_buffer_object(ctx, obj);
 }