panfrost: Move the fence creation in panfrost_flush()

panfrost: Move the fence creation in panfrost_flush()

panfrost_flush() is about to be reworked to flush all pending batches,
but we want the fence to block on the last one. Let's move the fence
creation logic in panfrost_flush() to prepare for this situation.

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