https://bugs.freedesktop.org/show_bug.cgi?id=107072
--- Comment #8 from Clemens Eisserer linuxhippy@gmail.com --- amdgpu.dc=0 did the trick - behaviour is back to normal.
I guzess it is not related to bug 106959. - kaveri doesn't support HDMI2, so my refresh rate was 30Hz even with the old display stack.