gallivm/swr: add stream_id to geom epilogue emit

gallivm/swr: add stream_id to geom epilogue emit

We want to pass a stream in here so we can write out separate
prim/vertex counts for each stream at the end.

This also adds an ignore any stream option so we can stage more code

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3530>