drm/virtio: add fence sanity check

Make sure we don't leak half-initialized fences outside the driver.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20190829103301.3539-19-kraxel@redhat.com