subsurface: fix changing stack order of sub-surface with its parent.

subsurface: fix changing stack order of sub-surface with its parent.

Sub-surface can be placed above or below of its parent by passing its
sub-surface with its parent as arguments to
wl_subsurface_place_above/below according to wl_subsurface description.

However, until now enlightenment seems not to have been supported this
operation correctly. In other words, enlightenment wasn't able to handle
stacking order of sub-surface in case wl_subsurface_place_above/below is
required with a parent of sub-surface.
This patch corrects it.

Change-Id: Icd65bf1759f1c7a3b950f6c9c222a41c9817a8ae