Hello Friend
I also have a x220 i7 (4290-58G i7-2640m) and I ran into the same problem. The solution found is: in clover in smbios set macbookpro8,1, and i set the year and week of manufacture (do not know if it affects, but I have a previous installation set only product name, but it did not work) and started working DP. The only problem - can not be the image on the monitor, then the monitor should be turned off and on.
I am sorry for english, google translate