drm/amd/powerplay: support VCN powergate status retrieval for SW SMU

Commonly used for VCN powergate status retrieval for SW SMU.

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>