svga: Add support for requesting NIR and translating to TGSI.

I'm working on switching mesa/st to no longer produce TGSI on its own, and so we need a way to test SVGA against that future. I don't have any way to test this patch myself. I expect there will be some tuning of the compiler options.

Anyone from VMWare able to pick this up and see what it takes to get this path working for you?