drm/amd/powerplay: support enabled ppfeatures retrieving and setting V3

User can use "ppfeatures" sysfs interface to retrieve and set enabled
powerplay features.

V2: expose this feature for Vega10 and later dGPUs
V3: squash in removal of unused variable (Alex)

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>