drm/amd/pm: drop the cache for enabled ppfeatures

The following scenarios make the driver cache for enabled ppfeatures
outdated and invalid:
  - Other tools interact with PMFW to change the enabled ppfeatures.
  - PMFW may enable/disable some features behind driver's back. E.g.
    for sienna_cichild, on gfxoff entering, PMFW will disable gfx
    related DPM features. All those are performed without driver's
    notice.
Also considering driver does not actually interact with PMFW such
frequently, the benefit brought by such cache is very limited.

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>