Comment # 6 on bug 99970 from Michel Dänzer

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.