platform/x86: fujitsu-laptop: sync backlight power status in acpi_fujitsu_laptop_add()

Registering an ACPI driver does not mean the device it handles has to
exist.  As the code which syncs backlight power status uses
call_fext_func(), it needs the FUJ02E3 ACPI device to be present, so
ensure that code is only run once the FUJ02E3 device is detected.

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>