power: adapt to GnomeRR API changes

Reading the backlight from GnomeRR can't fail anymore (although
-1 can be returned, meaning that XBACKLIGHT is not supported).
Also, for GnomeRR the value is always a percentage.

We still need to handle min/max in case we use sysfs directly
(which will be the common case when we move to wayland)

https://bugzilla.gnome.org/show_bug.cgi?id=705507