drm/amdgpu: Enable/disable gfx PG feature in rlc safe mode

This is required by gfx hw and can fix the rlc hang when
do s3 stree test on Cz/St.

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