panfrost: Prepare panfrost_fence for batch pipelining

panfrost: Prepare panfrost_fence for batch pipelining

The panfrost_fence logic currently waits on the last submitted batch,
but the batch serialization that was enforced in
panfrost_batch_submit() is about to go away, allowing for several
batches to be pipelined, and the last submitted one is not necessarily
the one that will finish last.

We need to make sure the fence logic waits on all flushed batches, not
only the last one.

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