after hours of bisecting I found the offending commit:
<http://cgit.freedesktop.org/xorg/driver/xf86-video-intel/commit/?id=19c48d3b3f33582baa87a9b3a9189e320e4cea45> breaks my backlight control. The light is then fixed at the lowest possible brightness.
Kernel bug - some of the backlight control paths do not respect the legacy mode.
*** This bug has been marked as a duplicate of bug 29716 ***