exynos4x12: liblights: Fix tablet with button backlight

based on
https://github.com/kumajaya/android_hardware_samsung/commit/a378cb1003eb325c3cc5e04e13dee4594134c48d
and exynos4x12: "liblights: Avoid duplicate declarations"
https://github.com/kumajaya/android_hardware_samsung/commit/40782eca301cddbb53ab9c4713e9b5432f95cf2d