msi-wmi: depend on backlight and fix corner-cases problems

Now depends on BACKLIGHT_CLASS_DEVICE.
Driver will return an error if it can't get actual backlight value
Fix remapping of brightness keys when backlight is not controlled by ACPI.

Signed-off-by: Anisse Astier <anisse@astier.eu>
Signed-off-by: Len Brown <len.brown@intel.com>