Comment 56 for bug 34155

That sounds like bad problem with DFS or UTS. You can disable them in xorg.conf with exa optinos to device context. More details are in man exa to what can be put to xorg.conf.

Can you attach dmesg and xorg.log when modeset=1 is passed to kernel? It works but kernel doesn't know about module to handle it before radeon is loaded so you get that warning for working parameter.