[server] Small code cleanup in SurfaceInterface

Summary:
Use lambda function to reduce code duplication and put
function definitions in the same order as in the header file.

Test Plan: All autotests succeed.

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: davidedmundson, kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D13191