virtio-gpu: update old resource too.

When switching scanout from one resource to another we must update the
scanout_bitmask field for both new (set bit) and old (clear bit)
resource.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20180702162443.16796-3-kraxel@redhat.com