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.

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. 😸

Closes: mesa#3532


Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <mesa!6997>