nir_to_tgsi: Add a workaround for virgl UBO array dynamic indexing.

nir_to_tgsi: Add a workaround for virgl UBO array dynamic indexing.

virgl makes one array of UBOs starting from the first non-CB0 UBO used,
and does dynamic indexing off of that.  It requires that the dynamic
indexing be CONST[ADDR[0]+base], rather than having the base be loaded in
addr0.

If we had a nir_intrinsic_base() on load_ubo, this would be easy.  As we
don't, emit a subtract at address deref time.

Acked-by: Gert Wollny <gert.wollny@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12800>