Good news! Not a solution, but a potential workaround.
I came across a report where someone with a lenovo system still needed to specify acpi_backlight=vendor. It might be worth a shot just to see.
Otherwise, according to that website, KDE integration hasn't been developed yet, so you may have to write your own scripts for now. I would suggest adding to /etc/rc.local (before the last "exit 0" statement) the following:
This would set the rights to this file so that everyone can write to it (meaning no need for sudo) and the second command would set a default brightness value that you would be comfortable with.
chmod 777 /sys/class/backlight/intel_backlight/brightness
echo some_value > /sys/class/backlight/intel_backlight/brightness
You can then use a script like this:
...to change the brightness (make sure you adjust the values of UPPER, LOWER, INCREMENT, DECREMENT so they work for you) and run brightness up or brightness down when you want to change the brightness. You can also try to map your current FN function keys to this script.
# wrapper script to change brightness values
# make script executable
# constants - edit as required
# current value
case $1 in
if [ $(($CURRENT+$INCREMENT)) -le $UPPER ]; then
if [ $(($CURRENT-$DECREMENT)) -ge $LOWER ]; then
# set the new brightness value
echo $NEW > /sys/class/backlight/intel_backlight/brightness