nir/xfb: move varyings info out of nir_xfb_info

nir/xfb: move varyings info out of nir_xfb_info

When varyings was added we moved to use to dynamycally allocated
pointers, instead of allocating just one block for everything. That
breaks some assumptions of some vulkan drivers (like anv), that make
serialization and copying easier. And at the same time, varyings are
not needed for vulkan.

So this commit moves them out. Although it seems a little an overkill,
fixing the anv side would require a similar, or more, changes, so in
the end it is about to decide where do we want to put our effort.

v2: (from Jason review)
  * Don't use a temp variable on the _create methods, just return
    result of rzalloc_size
  * Wrap some lines too long.

Fixes: cf0b2ad486c9 ("nir/xfb: adding varyings on nir_xfb_info and gather_info")
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>