Here is a tip for everyone with DP and DVI issues. I just upgraded to a 3 monitor Eyefinity setup. My monitors are connected to my HIS Radeon HD7950 via 2x active MiniDP to DVI and 1x DVI (also on my 2nd HD7950 I have DVI out to my HDTV). I won't go over my problems because they mirror all of you folk's experiences (and headaches). It has nothing to do with what frame buffer is used.
Enable IGPU in the bios, regardless if you will be using it or not. Problem solved! I can have all ports on both my HD7950 with an active monitor with ZERO issues. The moment I disable IGPU all hell breaks loose (again).
For users of Clover you will have no issues whatsoever.
For users of Chameleon/Chimera you will need to include the IGPU boot flag and set boot display to IGPU from PEG. If you are not using the IGPU this will be somewhat cumbersome because you will essentially be doing a 'dark boot', and if you have dual boot very annoying (remove the timer and guess when the boot loader is at the drive select screen), but it works.
Hope this helps.