https://bugs.freedesktop.org/show_bug.cgi?id=100289
--- Comment #10 from Julien Isorce julien.isorce@gmail.com --- It could be that drm.debug=0x01 will tell you if the error comes from one of the two "case:" here https://cgit.freedesktop.org/~agd5f/linux/tree/drivers/gpu/drm/drm_plane.c?h... . If it is the case it should appear in dmesg.
Also worth a try with https://launchpad.net/~paulo-miguel-dias/+archive/ubuntu/mesa with contains an up to date version of xf86-video-ati.
The error message is different but I wonder if the patch attached to https://bugs.freedesktop.org/show_bug.cgi?id=99769 would help here too.