turnip: xfb fixes and a bit of refactoring
It turns out XFB in turnip was quite broken (and passing most of the deqp tests because they are very basic), this fixes the remaining fails and refactors it a bit to be better. Fixes these tests:
dEQP-VK.transform_feedback.simple.backward_dependency*
dEQP-VK.transform_feedback.simple.draw_indirect*
(note the XFB is at least still a little broken, but it passes all the deqp tests)