Comment 22 for bug 411294

eading http://airlied.livejournal.com/68550.html from
describes how the bug was fixed.
Link to the fix:
http://people.freedesktop.org/~airlied/scratch/0002-drm-radeon-kms-read-back-register-before-writing-in-.patch
The bug can also be found in redhats bugzilla:
https://bugzilla.redhat.com/show_bug.cgi?id=527874
where it is marked as fixed. Unfortunately that is not where non-redhat/fedora
users would look :-(

I can confirm that this fixes my problem.
Not closing it yet as it isn't part of the kernel.org sources yet.