subsurface: Handles restack event only for topmost parent of sub-surface.

subsurface: Handles restack event only for topmost parent of sub-surface.

This patch removes a handler of restack event in e_comp_wl, and adds it
for topmost parent of sub-surface.
Because it's not necessary to handle restack event for sub-surface.
The reason is as follow.

1. Sub-surface can be re-stacked only by a request from wl_client. In
   other words, it shouldn't be re-stacked by compositor policy.
2. A topmost parent of sub-surface can be re-stacked only by compositor
   policy.

For these reason, it's sufficient for sub-surface implementation to add
a handler of restack event for topmost parent.

Change-Id: I6874425fd864c83039911dcf4ece75e5d2c8a79b