[wayland] Don't update margins in ShellClient::finishInit

Summary:
It doesn't make sense to update window margins in finishInit because no
buffer is attached yet at that moment.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D23743