backlight_helper_get_best_backlight()

[Jack Bates]

This patch was merged, thanks!
https://bugzilla.xfce.org/show_bug.cgi?id=12553

I'd like to follow up by incorporating this change as well:
https://git.gnome.org/browse/gnome-settings-daemon/commit/plugins/power/gsd-backlight-helper.c?id=9251341f0a3442d815a66ac9bd8be05322a1e1c9

I think it cleans up the change that was just merged by exploiting 
libgudev. Is this acceptable? Shall I attach a patch to a new issue?