sony-laptop: implement new backlight control method

Reasonably recent Vaios have a 0x12f or 0x137 handler that exposes a
fine lid backlight regulation with values ranging from 0 to 255.
The patch is based on findings and code from Javier Achirica
<achirica@gmail.com> and Marco Chiappero <marco@absence.it>

Signed-off-by: Mattia Dongili <malattia@linux.it>
Signed-off-by: Matthew Garrett <mjg@redhat.com>