drm/virtgpu api: cross-device feature

This feature was recently added to virtio-gpu, lets make
it userspace queryable. It's an error to use
BLOB_FLAG_USE_CROSS_DEVICE when this feature is not present.

Signed-off-by: Gurchetan Singh <gurchetansingh@chromium.org>
Acked-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200924003214.662-7-gurchetansingh@chromium.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>