drm/amd/powerplay: correct the APIs' naming

'UVD' is a HW engine name for Vega20 and before ASICs.
For newer ASICs, the similar engine is named as 'VCN'.

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