mesa/st: protect internal flushes with FLUSH_CURRENT

Already doing this for driver.flush()

src/mesa/state_tracker/st_cb_flush.c

@@ -82,7 +82,7 @@ display_front_buffer(struct st_context *st)
 void st_flush( struct st_context *st, uint pipeFlushFlags,
                struct pipe_fence_handle **fence )
 {
-   FLUSH_VERTICES(st->ctx, 0);
+   FLUSH_CURRENT(st->ctx, 0);
 
    /* Release any vertex buffers that might potentially be accessed in
     * successive frames: