drm/amd/pm: correct DPM_XGMI/VCN_DPM feature name

the following feature is wrong, it will cause sysnode of pp_features show error:
1. DPM_XGMI
2. VCN_DPM

Signed-off-by: Kevin Wang <kevin1.wang@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>