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

commit 8f612b23a17dce86fef75407e698de6243cc99a1
Author: Alex Deucher <alexander.deucher@amd.com>
Date:   Mon Mar 11 19:28:39 2013 -0400

    drm/radeon: fix backend map setup on 1 RB trinity boards