drm/amd/pm: clean up get_allowed_feature_mask function

The get_allowed_feature_mask is superfluous on vangogh.

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