I was bisecting over commits touching drivers/gpu/ only, but the one I found
seems pretty likely:

eccea7920cfb009c2fa40e9ecdce8c36f61cab66 is the first bad commit
commit eccea7920cfb009c2fa40e9ecdce8c36f61cab66
Author: Alex Deucher <alexander.deucher@amd.com>
Date:   Mon Mar 26 15:12:54 2012 -0400

    drm/radeon/kms: improve bpc handling (v2)

    Improve handling of bpc (bits per color) in radeon.
    In most cases we want 8 except for HDMI, DP, LVDS, and eDP.

    v2: handle DP better.

    Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
    Tested-by: Lennert Buytenhek <buytenh@wantstofly.org>
    Signed-off-by: Dave Airlie <airlied@redhat.com>

I'll verify that further tomorrow.