platform samsung-q10: use ACPI instead of direct EC calls

This patch changes the Samsung Q10 backlight driver to use ACPI methods
(the same ones as triggered by the brightness up/down function keys)
instead of direct EC calls. The advantage is that the brightness setting
is not lost on shutdown.

Signed-off-by: Frederick van der Wyck <fvanderwyck@gmail.com>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>