https://bugs.freedesktop.org/show_bug.cgi?id=80029
--- Comment #14 from Mario Kleiner mario.kleiner@tuebingen.mpg.de --- Ok, there's nothing suspicious in your edid's or syslog output, everything seems to work according to plan.
Assuming your equipment is working correctly, this leaves dce8 specific setup as suspect. Alex used some special case code for dce5 and dce6, but excluded dce8 from that special case. However we haven't ever tested dce8 + deep color, so maybe it needs that special case as well.
Can you apply and try the patch i just attached? This is a pure guess, but maybe it helps.