[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