drm/virtio: Don't return invalid caps on timeout

If the wait timeouts, the caps are probably invalid and we shouldn't be
passing them to userspace.

Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20171127142126.25765-1-tomeu.vizoso@collabora.com


Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>