Bug#765551: All 3 monitors show a black screen on GDM start when OneLink Pro dock is connected to Lenovo X1 Carbon

[Benjamin Xiao]

Package: gdm3
Version: 3.13.92-1

I have a Lenovo X1 Carbon Type 20A8 laptop with a OneLink Pro dock. I
have two external monitors, one connected to the mini-DisplayPort on
the laptop itself and another connected to the DVI port on the dock.
Under Ubuntu 14.04, I am able to drive all 3 displays (laptop built-in
display and the two external monitors) just fine.

Under Debian the behavior is quite weird:

*If I have the dock plugged in before boot, all 3 screens are black
once GDM launches into the login screen.

*If I have the dock plugged in after boot and all 3 displays are
enabled in the GNOME Display settings, then everything turns black,
but I can see the mouse cursor moving around.

*If I plug in one of the monitors to the mini-DisplayPort, disable the
laptop built-in display, and THEN plug in the dock, I can drive the
two external screens fine.

Expected behavior:
I should be able to drive all 3 displays just fine and unplug each
external monitor and have it dynamically readjust.

The Xorg log looked okay to my untrained eye. I am unsure of which
logs to attach. Please let me know which ones you guys need to debug
this and I will be happy to get it to you ASAP.

I am using an up to date Debian testing with kernel 3.16-2-amd64 and
libc6 2.19-11