panvk: Make panvk_cmd_open_batch() return the new batch

panvk: Make panvk_cmd_open_batch() return the new batch

So we can get replace the

   panvk_cmd_open_batch();
   batch = cmdbuf->state.batch;

sequence by

   batch = panvk_cmd_open_batch();

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13077>