drm/amdgpu: Split set_pg_state into separate function

1. add amdgpu_device_ip_late_set_pg_state function for
   set pg state.
2. delete duplicate pg state setting on gfx_v8_0's late_init.

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