drm/virtio: implement context init: advertise feature to userspace

This advertises the context init feature to userspace, along with
a mask of supported capabilities.

Signed-off-by: Gurchetan Singh <gurchetansingh@chromium.org>
Acked-by: Lingfeng Yang <lfy@google.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20210921232024.817-13-gurchetansingh@chromium.org


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