platform/x86: fujitsu-laptop: use device-specific data in backlight code

To prevent using module-wide data in backlight-related code, employ
acpi_driver_data() and bl_get_data() where possible to fetch
device-specific data to work on in each function.  This makes the input
local variable in acpi_fujitsu_bl_notify() and the acpi_handle field of
struct fujitsu_bl redundant, so remove them.

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