exynos4x12: liblights: Fix tablets with button backlight

Also, respect the lights API and return errors when attempting to
configure an absent light ID.

Change-Id: Ida430fba2fbfaf646f7e02ce2bd7d9134ae8d917
Merge branch 'cm-11.0' of https://github.com/CyanogenMod/android_hardware_samsung into cm-11.0