panfrost: Allocate syncobjs in panfrost_flush

panfrost: Allocate syncobjs in panfrost_flush

For implementing panfrost_flush, it suffices to wait on only a single
syncobj, not an entire array of them. This lets us wait on it directly,
without coercing to/from syncfds in the middle (although some complexity
may be added later to support Android winsys).

Further, we should let the fence own the syncobj, tying together the
lifetimes and thus removing the connection between syncobjs and
batch_fence.

Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5995>