platform/x86: fujitsu-laptop: use a sparse keymap for brightness key events

Simplify brightness key event generation by using a sparse keymap.

Signed-off-by: Michał Kępień <kernel@kempniu.pl>
Reviewed-by: Jonathan Woithe <jwoithe@just42.net>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>