gallivm/draw/swr: make the gs_iface not depend on tgsi.

gallivm/draw/swr: make the gs_iface not depend on tgsi.

This gs_iface doesn't seem to require a dependence on the tgsi
context, except for the swr end prim code.

This refactors the API to include all the info that the swr
code needs in the interface rather than having to dig it out of
the struct inheritance.

This is a precursor to adding NIR support to llvmpipe.

Reviewed-by: Jan Zielinski <jan.zielinski@intel.com>