pan/mdg: Allow Z/S writes to use any 2nd stage unit

This ensures there will not be dependency problems if we emit a move
that tries to read from a parallel instruction.

No shader-db changes.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5513>
1 file changed