How to permanently disable (defective) graphics card in MacBook Pro?
I have had this same problem with this same card. I know that it won't boot when I see horizontal blue bars through the Apple logo at the login screen, it goes to white-screen after that and the fans rev up.

I've found the most reliable way to boot up is to let it advance to the white/grey screen after the Apple logo, then I close the lid and wrap it in a blanket or put it in my laptop sleeve. It will overheat and automatically shut down. Almost every time it will start up in that state. 

It used to be tense but after 9 months of this it's pretty standard.