drm/amdgpu: add VCN to firmware query interface

Need to be able to query the VCN firmware version from
userspace to determine supported features, etc.

Reviewed-by: Huang Rui <ray.huang@amd.com>
Acked-by: Leo Liu <leo.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>