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

include/uapi/linux/virtio_gpu.h

@@ -309,6 +309,8 @@ struct virtio_gpu_cmd_submit {
 
 #define VIRTIO_GPU_CAPSET_VIRGL 1
 #define VIRTIO_GPU_CAPSET_VIRGL2 2
+/* 3 is reserved for gfxstream */
+#define VIRTIO_GPU_CAPSET_VENUS 4
 
 /* VIRTIO_GPU_CMD_GET_CAPSET_INFO */
 struct virtio_gpu_get_capset_info {