I just want to control brightness!

OK, sure. Here are some hints, which might help to you to get things working.

1. First of all, make sure that you've read appropriate chapters in the FreeBSD handbook:
   • Chapter 5. The X Window System: 5.4. Xorg Configuration
2. Check out which driver you need based on the information presented on the Graphics wiki page. Use pciconf -lv to figure out what hardware you have (see man 8 pciconf for details).

• The order in which you install the Intel xf86-video-intel driver and the Nvidia driver matters! Reinstall them when in doubt.
• Running Xorg -configure is not recommended. It rarely helps. Remember to remove those xorg.conf files you've created this way if you are still debugging.

Graphics/OptimusVideoSupport

Graphics/SCFB

• This driver does not support brightness control as of 2018/03/18.

• Sometimes you may need to load the acpi_video(4) kernel module to use the function keys to control brightness.

• Brightness Function Keys Issue (forums.freebsd.org)

Screen temperature, gamma and backlight related software is also welcome in this section.

• intel_backlight
• redshift
• sct
  • As of sct 0.2.1 there seems to be some issues with i386 machines. Try Redshift if sct doesn't work on your machine.
• X server utilities:
  • xbacklight
  • xbrightness
  • xgamma
  • xrandr (e.g. xrandr --output eDP-1 --brightness 0.5)

• Channels:
  • #freebsd-xorg on EFNet
  • #freebsd on Freenode

• Brightness control in FreeBSD 10.3 RELEASE on Dell XPS L702X (superuser.com)
• X