Comment 6 for bug 1376581

I have the same issue.
My configuration:
- Ubuntu 14.04, with all updates (Ubuntu 3.13.0-37.64-generic 3.13.11.7)
- EliteBook 840 G1 connected to a HP ultraslim docking station 2013.
- 1 Acer B273HU connected to the DP of the docking

FGLRX: linux-amd-catalyst-14.6-beta-v1.0-jul11.zip: fglrx-14.20
(I had to install the following packages:
sudo apt-get install dh-modaliases execstack dkms lib32gcc1 build-essential xserver-xorg-dev
Before I could then finally build the package:
~/Downloads/fglrx-14.20$ sudo ./amd-driver-installer-14.20-x86.x86_64.run --buildpkg Ubuntu/trusty
And install it:
~/Downloads/fglrx-14.20$ sudo dpkg -i fglrx_*.deb fglrx-amdcccle_*.deb
)

When connecting the monitor to the DP of the Laptop everything works fine.
When connecting the monitor to the DP of the Docking Station nothing happens.

One more thing to mention is that when using the official driver: "Using X.Org X server - AMD/ATI display driver wrapper from xserver-xorg-video-ati (open source, tested)"
Then the DP of the docking station works fine (but the rendering on the external monitor is ugly: tearing, artifacts, etc.)