62444b7462a2b98bc78d68736c03a7c4e66ba7e2 is the first bad commit
commit 62444b7462a2b98bc78d68736c03a7c4e66ba7e2
Author: Alex Deucher <alexander.deucher@amd.com>
Date:   Wed Aug 15 17:18:42 2012 -0400

    drm/radeon: properly handle mc_stop/mc_resume on evergreen+ (v2)

    - Stop the displays from accessing the FB
    - Block CPU access
    - Turn off MC client access

    This should fix issues some users have seen, especially
    with UEFI, when changing the MC FB location that result
    in hangs or display corruption.

    v2: fix crtc enabled check noticed by Luca Tettamanti

    Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

:040000 040000 3e0d33c9b4eda29ced814fe9a863efe63e53f14c
4932561607b160734ec1eade927a9fe18c9f3f1b M    drivers

So in other words, your explanation Alex seems to be right. I'll be waiting if
anything has to be tested.