drm/amd/powerplay: refine code in amd_powerplay.c (v2)

1. use flag PP_DPM_DISABLED within powerplay
   notify amdgpu dpm state by cgs interface.
2. delete redundant virtualization check in
   powerplay

v2: squash in fix for hwmgr_init (Rex)

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