platform/x86: fujitsu-laptop: switch to a managed backlight device

Use a managed backlight device to get rid of acpi_fujitsu_bl_remove().
Change the parent of the backlight device from NULL to the FUJ02B1 ACPI
device as the latter is required for the backlight device to work.

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