Unfortunately, same situation. I have DVI HDMI working. It doesn't recognize DP. Maybe I have syntax mistakes in config.plist? Or wrong bios settings.. I have vd-t - enable, intel graphics - disable, CSM - as I understood enable. (I have yosemite 10.10.3 beta 2 already).
I had to enable CSM to get DP to work. I bought a cheap HDMI --> VGA adapter so I could turn CSM off. Previously I only had a DP --> VGA adapter.