units: Do not unescape instance name in systemd-backlight@.service

The instance name is never escaped in the udev rule, but unescaped in the unit.
This results in the following error message on Asus boards:

  Failed to get backlight or LED device 'backlight:eeepc/wmi': No such file or directory

(cherry picked from commit 6c492127)
(cherry picked from commit 50ab7a79)