broadcom/vc5: Fix transform feedback in the presence of point size.

broadcom/vc5: Fix transform feedback in the presence of point size.

I had this note to myself, and it turns out that a lot of CTS tests use
XFB with points to get data out without using a fragment shader.  Keep
track of two sets of precomputed TF specs (point size in VPM prologue or
not), and switch between them when we enable/disable point size.