linuxkpi: Add backlight_device_set_brightness

This simply set the brightness of a backlight device.
Needed by drm-kmod v5.8

Reviewed by: bz, emaste
MFC after: 2 weeks
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D34285

(cherry picked from commit 2cc3af6e1d795c6d33afaf994c18abaab87a443b)