graphics/intel-backlight: Deprecate the port

Now that FreeBSD 12.X is EoLed every FreeBSD version have the backlight
subsystem. backlight(8) works with any Intel or AMD hardware as long as drm-kmod
is loaded.

Differential Review:	https://reviews.freebsd.org/D44194
Approved by:		maintainer timeout (1 month)
Sponsored by:		Beckhoff Automation GmbH & Co. KG

(cherry picked from commit 30600771)