Comment # 1 on bug 1180591 from Takashi Iwai

Could you give the output of hwinfo and the output of dmesg?

Also, try to adjust the brightness by manually writing to sysfs files
/sys/class/backlight/*/brightness.  The value ranges are found in
/sys/class/backlight/*/max_brightness.  e.g. on my laptop, I can change it like
  echo -n 200 > /sys/class/backlight/intel_backlight/brightness

There might be multiple entries in /sys/class/backlight and you can try each of
them.

And, if it used to work with Leap kernel, could you try to install the latest
Leap 15.2 kernel on the current system (with zypper install --oldpackage
option), boot with it and test whether the brightness works?
If Leap 15.2 kernel works, give again the output of hwinfo and dmesg from it.