dc_stream_get_brightness_millinits_from_refresh
int lower_refresh_brightness = dc_stream_get_brightness_millinits_from_refresh (stream, hz1);
int higher_refresh_brightness = dc_stream_get_brightness_millinits_from_refresh (stream, hz2);
dc_stream_get_brightness_millinits_from_refresh(stream, current_refresh_hz),
int current_brightness = dc_stream_get_brightness_millinits_from_refresh(stream, starting_refresh_hz);
int current_brightness = dc_stream_get_brightness_millinits_from_refresh(stream, starting_refresh_hz);