virgl: don't mark unclean after a flush

The guest memory is still clean until host GL touches it,
which we should track elsewhere.

Reviewed-by: Gert Wollny <gert.wollny@collabora.com>

src/gallium/drivers/virgl/virgl_buffer.c

@@ -120,7 +120,6 @@ static void virgl_buffer_transfer_flush_region(struct pipe_context *ctx,
     * We'll end up flushing 25 --> 70.
     */
    util_range_add(&trans->range, box->x, box->x + box->width);
-   vbuf->clean[0] = FALSE;
 }
 
 static const struct u_resource_vtbl virgl_buffer_vtbl =