[platforms/wayland] Do an initial xdg-surface commit

Summary:
After setting the xdg-surface role an initial commit must be sent per
protocol description.

This fixes Wayland nested session after commit 84de8d13 made this
requirement explicit.

Test Plan: Launched Wayland nested session.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D25127