mesa: Port the pbo.use_gs path to NIR and let it get used on NIR drivers.

Now that everyone but SVGA is requesting NIR, this path had been
effectively disabled.  I had done a partial port of the VS side in
9143c08125a3 ("st/nir: Fix the st->pbo.use_gs case.") for the sake of
nv50, but with it should be ready for all drivers.  Affects nv50, v3d,
d3d12, svga (I think).

Note that this GS code is slightly different from the TGSI: We put a 0 in
pos.z, rather than leaving the layer value there, because apparently v3d
didn't like those denorm Z values.

Also, it's nice to see that the NIR code is shorter than the TGSI code
was, we've made great progress on nir_builder.

Reviewed-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Acked-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23114>

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