Comment # 10 on bug 798308 from Egbert Eich

Thanks for the log! 
The external monitor connected thru DP should be set to 1920x1080.
This is what xrandr should say as well. 

Ok, this is an issue that existed for several kernel versions and seemed to
work with fglrx once (at least on Ubuntu). This rules out a monitor or cable
problem.

@Alex - Alex, do you have an idea?