I run Ubuntu on my Lenovo P50, and the backlight keys haven’t ever worked. Here’s how I got it working.
sudo apt install xbacklight
Then I mapped the keys using Settings > Devices > Keyboard and added mappings for the following:
Windows-F5: xbacklight -inc 5 -time 1 -steps 1
Windows-F6: xbacklight -dec 5 -time 0 -steps 1