platform/x86: fujitsu-laptop: only handle backlight when appropriate

The backlight part of fujitsu-laptop is only used by laptops which are
incapable of using the standard ACPI video interface for handling
brightness changes.  Conversely, on laptops which are capable of using
the latter, no vendor-specific ACPI calls should be made unless
explicitly requested by the user.  Bail out immediately from
acpi_fujitsu_bl_add() unless using the vendor-specific interface was
either explicitly requested by the user or automatically selected by the
kernel.

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