platform/x86: fujitsu-laptop: account for backlight power when determining brightness

The comment for the get_brightness backlight device callback in
include/linux/backlight.h states that it should "return the current
backlight brightness (accounting for power, fb_blank etc.)".
fujitsu-laptop violates that premise by simply returning the value to
which ACPI function GBLL evaluates to.  Make sure 0 is returned when
backlight power is turned off.

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