pan/bi: Adapt builder to dest count

pan/bi: Adapt builder to dest count

If there are no destinations, don't produce a _to version, and let the
bare version return the bi_instr.

If there are multiple destinations, take each in the _to version and
don't produce a bare version.

Both cares are probably what you wanted anyway.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9164>