wayland_server: Improve debug message, reduce duplicates

Summary:
Demote the "interface not granted" message from a warning to debug
and improve the wording a bit.
We are listing all the interfaces, it's not like we're not granting an interface
that the application wants, it's that we're not offering an interface to an application
because it didn't opt in.
Only report every miss once, to reduce the noise.

Test Plan: ran kwin

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D29759