drm/amd/powerplay: add new Fiji function for not setting same ps.

Add comparison function used by powerplay to determine which
power state to select.

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