platform/x86: lenovo-yogabook: Abstract kbd backlight setting

Abstract kbd backlight setting.

This is a preparation patch for making lenovo-yogabook-wmi also work
on the Android version of the Yoga Book 1 which does not have a WMI
interface to deal with toggling the keyboard half between
touch-keyboard and wacom-digitizer mode.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20230430165807.472798-13-hdegoede@redhat.com