I'm sort of necrobumping here but I was trying to figure this out too and I've found that you can do it by writing a numeric value to /sys/class/backlight/pwm-backlight/brightness (keep in mind the max value set in the max_brightness file in the same folder).
I created two simple Bash scripts for raising and lowering the brightness via this file. I then bound the brightness keys to call those scripts when I press them (in dwm). The downside to this is that you have to change permissions on that brightness file to be writable by your normal user. Can anyone think of a safer option than that?
Edit: nevermind this solution, use mchack's solution below!