virgl: do not skip readback because of explicit flush

Both apps and we (see virgl_buffer_transfer_flush_region) might
flush regions that are unmodified.  We have to read back for those
flushes.

Signed-off-by: Chia-I Wu <olvaffe@gmail.com>
Reviewed-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>

src/gallium/drivers/virgl/virgl_resource.c

@@ -56,9 +56,6 @@ bool virgl_res_needs_readback(struct virgl_context *vctx,
       readback = false;
    else if (usage & PIPE_TRANSFER_DISCARD_RANGE)
       readback = false;
-   else if ((usage & (PIPE_TRANSFER_WRITE | PIPE_TRANSFER_FLUSH_EXPLICIT)) ==
-            (PIPE_TRANSFER_WRITE | PIPE_TRANSFER_FLUSH_EXPLICIT))
-      readback = false;
    return readback;
 }