Comment # 16 on bug 1213380 from Samuel Conway

(In reply to Takashi Iwai from comment #14)
> So, now you could install the system with DP-DP?  And on the running system,
> try to switch to DP-HDMI.  Do you see the same problem?  When you reconnect
> back to DP-DP, does it recover again?
> 
> If all yes, please try like the following:
> - Boot with DP-DP, go up to GUI desktop
> - Enable the debugging of drm, run below as root
>     echo 0x1e > /sys/module/drm/parameters/debug
> - Re-connect to DP-HDMI, and you'll see a buggy screen
> - Re-connect back to DP-DP, screen recovered
> - Save the output "dmesg" command, and attach the output to Bugzilla

Did as as requested, had video output for both DP-DP and DP-HDMI. output of
dmesg  is attached.