panfrost: Add a panfrost_flush_all_batches() helper

panfrost: Add a panfrost_flush_all_batches() helper

And use it in panfrost_flush() to flush all batches, and not only the
one currently bound to the context.

We also replace all internal calls to panfrost_flush() by
panfrost_flush_all_batches() ones.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>