A patch referencing this bug report has been merged in Linux v3.7-rc1:

commit bced76f27165ca7733437715185c3a1aa526f7a1
Author: Alex Deucher <alexander.deucher@amd.com>
Date:   Fri Sep 14 09:45:50 2012 -0400

    drm/radeon: restore backlight level on resume