platform/x86: fujitsu-laptop: only register backlight device if FUJ02B1 is present

As the backlight device registered by fujitsu-laptop relies on the
FUJ02B1 ACPI device being present, only register the backlight device
once that ACPI device is detected.

Suggested-by: Alan Jenkins <alan-jenkins@tuffmail.co.uk>
Signed-off-by: Michał Kępień <kernel@kempniu.pl>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-and-tested-by: Jonathan Woithe <jwoithe@just42.net>