A patch referencing this bug report has been merged in Linux v3.8-rc1:

commit bd25f0783dc3fb72e1e2779c2b99b2d34b67fa8a
Author: Jerome Glisse <jglisse@redhat.com>
Date:   Tue Dec 11 11:56:52 2012 -0500

    drm/radeon: fix amd afusion gpu setup aka sumo v2