drm/virtio: add definition for venus capset

This Venus capset definition is used by Qemu, and Qemu imports the
kernel protocol header file. Add Venus capset to the VirtIO-GPU protocol.

Signed-off-by: Huang Rui <ray.huang@amd.com>
[dmitry.osipenko@collabora.com: edit commit message]
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230915105918.3763061-1-ray.huang@amd.com