drm/amdgpu/powerplay/vega10: enable AVFS control via ppfeaturemask

Allow the user to disable AFVS via ppfeaturemask for debugging.

Reviewed-by: Rex Zhu <Rex.Zhu@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>