csd-power: Refactoring display backlight code.

- Work with percentages instead of absolute values.
- If gnome-rr fails, use the backlight-helper automatically.
- Remember the failure of gnome-rr so the backlight-helper
  is given priority for future calls.

The backlight-helper-force setting is now only useful if gnome-rr
calls do not behave properly, but do not actually fail either.
In this case they can be force-skipped like before.

Author: mtwebster