A patch referencing this bug report has been merged in Linux v3.8-rc7:

commit ed39fadd6df01095378e499fac3674883f16b853
Author: Alex Deucher <alexander.deucher@amd.com>
Date:   Thu Jan 31 09:00:52 2013 -0500

    drm/radeon/evergreen+: wait for the MC to settle after MC blackout