A patch referencing this bug report has been merged in Linux v3.9-rc4:

commit fa3daf9aa74a3ac1c87d8188a43d283d06720032
Author: Alex Deucher <alexander.deucher@amd.com>
Date:   Mon Mar 11 15:32:26 2013 -0400

    drm/radeon: fix S/R on VM systems (cayman/TN/SI)