drm/amd/powerplay: correct UVD/VCE/VCN power status retrieval

VCN should be used for Vega20 later ASICs while UVD and VCE
are for previous ASICs.

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>