This bug in EDID was exposed by:

commit eccea792
Author: Alex Deucher <alexander.deucher@amd.com>
Date:   Mon Mar 26 15:12:54 2012 -0400

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

Which resulted in kind of regression in 3.5. This fixes
https://bugs.freedesktop.org/show_bug.cgi?id=70934Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>