drm/amd/powerplay: add new sensor type for VCN powergate status

VCN is widely used in new ASICs and different from tranditional
UVD and VCE.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Kenneth Feng <kenneth.feng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>