panfrost: Use the per-batch fences to wait on the last submitted batch

panfrost: Use the per-batch fences to wait on the last submitted batch

We just replace the per-context out_sync object by a pointer to the
the fence of the last last submitted batch. Pipelining of batches will
come later.

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