platform/x86: fujitsu-laptop: register backlight device in a separate function

Move code responsible for backlight device registration to a separate
function in order to simplify error handling and decrease indentation.
Simplify initialization of struct backlight_properties.

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>