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