backlight: fix issue on multiple graphics cards system

backlight: fix issue on multiple graphics cards system

If a system has multiple graphics cards, then we cannot associate
platform backlight devices to backlight devices under PCI bus.

Previously, in such case, vaidate_device() for a raw backlight device
might erroneously detect a platform device and return false. So, users
could not save/load backlight level.

This makes validate_device() give up to associate platform devices on
non-PCI bus with raw backlight devices. That may cause unwanted
backlight level save or restore by systemd-backlight@.service, but users
can workaround that by masking specific instances of the service.

Closes #24223.