sony-laptop: notify userspace of GFX switch position changes

Some Vaios come with both integrated and discrete graphics, plus a
switch for choosing one of the two. When the switch position is changed,
a notification is generated.

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