drm/amd/powerplay: refine avfs enable code on fiji.

1. simplify avfs state switch.
2. delete save/restore VFT table functions as not support
   by fiji.
3. implement thermal_avfs_enable funciton.

Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>