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 swr end prim code, which seems like it could be done better).

This just makes it use the lp_build_context, which can be upcasted if the end driver always consumes tgsi (like swr), it also drops the unused bld pointer from gs_epilogue.