There's a generic udev rule to set group ownership of backlights to
'video', but it only runs on the ADD action, which seems to be too
late(?) since the backlight device is never owned by 'video' on boot.
This rule will run on CHANGE too, which might be overkill but seems to
get the job done. It's now possible to set the backlight brightness on
the PBP as a normal user.
