[server] Don't make assumptions about the order of damage_buffer and attach requests

Summary:
The spec says nothing about the order between damage_buffer and attach
requests.

Test Plan: Firefox doesn't become frozen. Although there are still issues with resizing.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D28147