gallium: call draw_flush() in softpipe_flush()

Without this, we might not get any rendering at SwapBuffers time when using the vbuf path.

src/mesa/pipe/softpipe/sp_flush.c

@@ -50,6 +50,8 @@ softpipe_flush( struct pipe_context *pipe,
    struct softpipe_context *softpipe = softpipe_context(pipe);
    uint i;
 
+   draw_flush(softpipe->draw);
+
    /* - flush the quad pipeline
     * - flush the texture cache
     * - flush the render cache