Black screen installation mac os on "non mac edition" graphic card

Hello,

First of all i'm sorry if my discussion isn't very clear my situation is a bit complex :

I have a mid 2010 Mac Pro, on which i changed my original graphic card to a Gtx 980.

I want to precise that i don't have my original graphic card a Radeon 5870.

I recently bought an SSD and wanted to install Mac os Yosémite on it.

My 1st intention was to install it using a boot through Time Machine but it did not work since i cant use Recovery HD anymore, because of my non "mac edition" graphic card.

I gave up this idea and tried to installed Yosémite on the SSD by downloading the installer directly from the Apple Store.

I lauched the installer followed the installation and when it asked me to restart my computer to continue, i clicked yes.

But when i restarted my computer, i got stuck on a black screen the installation launcher doesn't start nothing happens and i cant start my computer anymore.

I understand why : Because i don't have a certified "mac edition" graphic card, but now i'm completely stuck, cannot do anything, i tried to reset the NVRAM

but that didn't work aswell.

So i would like to know if any of you have ever deal with this problem and if you have a solution ?

I can see a solution but i would like avoiding it : i think i have to buy a mac edition graphic card in order to complete the installation, because it will have a boot screen, but i would like to know if you have another one.

Thanks in advance,

My apoligies for my Bad English, i tried my best !