Hi, I'm wanting to get a Dell U2913WM 29" 2560x1080 display and connect it to my 2011 21.5 inch iMac (i5 2.7GHz, 6750M 512MB.) What kind of adaptor do I need for it? Do I need to find a Thunderbolt to HDMI 1.2 adaptor? Or what'll work? Would greatly appreciate any help!