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

commit 3646e4209f2bd0d09022ed792e594fb4f559b86c
Author: Alex Deucher <alexander.deucher@amd.com>
Date:   Thu Jan 31 16:19:19 2013 -0500

    drm/radeon: switch back to the CP ring for VM PT updates