I tried also the solutions from this thread: https://bbs.archlinux.org/viewtopic.php?id=151119
1. enabling brightness control in device section in /etc/X11/xorg.conf.d/20-nvidia.conf - no change in behaviour
2. change acpi_backlight to vendor - even the /sys/class/backlight/ is empty.
In the thread previously mentioned they claim everything works fine. Is there anybody with hp laptops experiencing the same behaviour?
Here is the kde bug report page: https://bugs.kde.org/show_bug.cgi?id=296403
They claim it works on archlinux now....