Implement back-light control for internal displays.

If system has 1 internal display and /sys/class/backlight/backlight/brightness, /sys/class/backlight/backlight/max_brightness is availabe / accessible, HAL API can be redirected to this sysfs entries.

This could be extended in the future using external configuration to map display to brightness controller.

weston reference: https://gitlab.freedesktop.org/wayland/weston/-/blob/master/libweston/backend-drm/libbacklight.c