subsurface: fix not being able to update stack by place_below_parent.

subsurface: fix not being able to update stack by place_below_parent.

This problem was caused by the following patch I made for the reason that
described in the commit message.

===
commit 3d028b1c348ab53170b4f6e70ebe0794de14ff8d
Author: Seunghun Lee <shiin.lee@samsung.com>
Date:   Thu Oct 22 13:41:18 2020 +0900

    subsurface: Do not update stacking order of entire sub-surface tree.
===

The stacking order of sub-surface should be actually updated when state
of its immediate parent is applied according to the wl_subsurface
description.

However, 'tizen_policy.subsurface_place_below_parent' seems not to be
designed like that. So, just let 'tizen_policy.subsurface_place_below_parent'
update the stacking order immediately.
Otherwise I think this protocol can't support backward comatibility.
Clients of wayland server must be assuming that changing stacking order by
this protocol will be applied regardless of state of its parent.

Change-Id: I11a7644d16431f612a5674851a40964a48d97bbf