gallivm: fix stream id fetch

gallivm: fix stream id fetch

Fetching the stream id directly can crash since bld->immediates may not
exist (if there's too many immediates or we use the array due to indirect
accesses). So just call emit_fetch_immediate instead.

v2: fix the swizzle

Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4416>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4416>