anv: Implement VK_EXT_transform_feedback on Gen7
Things work a little different on Gen7 than they do on Gen8+. In particular, SOBufferEnable lives in 3DSTATE_STREAMOUT but BufferPitch lives in 3DSTATE_SO_BUFFER. This leaves us having to marshal data around a bit more than we did on Gen8. Still, it's not too bad.
Closes: #3532 (closed)
Normally, I don't spend much time on Gen7 but XFB just became a hard requirement for DXVK so it stopped working for all our Haswell users. Let's get them happily playing their games again.