platform/x86: fujitsu-laptop: Define constants for backlight power control

Use named constants instead of integers in call_fext_func() invocations
related to backlight power control in order to more clearly convey the
intent of each call.

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