Thanks for your reply.
The adapter I use is not an active one. The DVI plug has all the pins like a dual link adapter (according to Wikipedia http://en.wikipedia.org/wiki/Digital_Visual_Interface
) but apparently not all of them are really used.
To make it short, I already used a DisplayPort to DVI-D (Single Link) adapter - the information in my initial post is wrong. Sorry about that.
But more important, I was able to get the second monitor up and running via DisplayPort. The problem was that I patched the kmod-nvidia from rpmfusion. The patch solves a problem with Intel-VT enabled and can be found here: http://www.nvnews.net/vbulletin/showthread.php?t=151791
After installing the original driver from rpmfusion and readding iommu=soft to the kernel boot line the DisplayPort works fine.
The patch solved the vt-iommu issue but it seems to break the DisplayPort. I think the problem is either the patch or the way I applied it to the package.
If you need more information feel free to ask.