Created attachment 5754 enable xrandr brightness control for eDP Backlight brightness control via xrandr API is not limited to LVDS displays but also available when the display is connected by Embedded Display Port (eDP).
Thanks for the patch. Fixed in http://git.xfce.org/xfce/xfce4-power-manager/commit/?id=4b3d97404d0b2d0bb3d677f719aa3bad2dfc13de