https://bugs.freedesktop.org/show_bug.cgi?id=83742
--- Comment #4 from Ralf-Peter Rohbeck RalfPeter.Rohbeck@quantum.com --- Resolution to the mystery: It seems that I had changed /etc/default/grub before today's testing but not run update-grub. When I installed the 3.13-0.bpo.1 kernel the kernel command lines were fixed.
What caused the original failure were the explicit mode settings I had on the kernel command line, namely "video=DP-1:2560x1440-24@60e video=DP-2:2560x1440-24@60e video=DVI-0:2560x1440-24@60e". I had to add those settings in the past (see https://bugs.freedesktop.org/show_bug.cgi?id=72167.) Without them settings it works.
I'll attach some kern.logs - there all sorts of interesting things going on that are apparently needed to get the DP outputs up and running with my DP->DVI converters. Setting the resolutions explicitly keeps those interesting things from happening.