September 30th, 2014 09:16 AM
Try booting with GraphicsEnabler=Yes.
We'll have to wait for the Yosemite release to tell. But, the 7950 works natively in Mavericks because there is a HD 7950 Mac Edition graphics card that Apple released for the Mac Pro (5,1).
BTW, with respect to discussion on Yosemite on this Forum, the Rules state: