Early 2011 Macbook Pro DisplayPort 1.2 driver
I had a similar compatibility issue a few years back with a Dell monitor.  After a replacement and multiple support calls, it ended up being the Mini DisplayPort to DisplayPort cable.  The embedded chip didn't send the proper info the Dell monitor is expecting.

After I replaced the Mini DisplayPort to DisplayPort cable with a Mini DisplayPort to DisplayPort adapter and a normal sized DisplayPort to DisplayPort cable, it worked properly.

There is some kind of incompatibility with Dell monitors and MDP to DP cables.  I've used the MDP to DP cable with other monitors without problems but have only had success with Dells when using a MDP to DP Adapter with a DP-DP cable.

Hope this helps!