https://bugs.freedesktop.org/show_bug.cgi?id=99970
--- Comment #6 from Michel Dänzer michel@daenzer.net --- This could be due to a fundamental incompatibility between DRI3 and EXA (as it is used by the radeon driver). That incompatibility is the reason for the radeon(4) manpage saying:
Note: DRI3 may not work correctly in all cases with EXA, enable at your own risk.
You can try glamor, but I'm not sure it can work with your GPU.