asahi: Fix batch writer tracking for null batches

asahi: Fix batch writer tracking for null batches

When an empty batch is submitted, nothing happens. However, this batch
may have taken over writer status for some BOs which still have a
pending submitted batch that hasn't finished yet. If we drop writer
status at this point, two bad things happen:

- We spuriously clear bo->writer_syncobj, which breaks syncing on
  post-facto BO exports
- We break agx_sync_writer(), since we no longer know about the old
  writer to properly block on it.

To fix this (hopefully rare) case, take advantage of bo->writer_syncobj
to find the currently submitted writer batch again, and revert the
writer to it. If this turns out to be common and a performance issue
iterating through submitted batches for each written BO, we could
implement it with two writer batch arrays instead, one for active
writers and one for submitted writers... but hopefully that isn't
necessary.

This splits the cleanup path in agx_batch_cleanup() depending on whether
the cleanup is for a reset or proper completion. Since this is only used
within agx_batch.c, drop the public prototype while we're at it.

Signed-off-by: Asahi Lina <lina@asahilina.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/22891>