[server] Use cached current mode more and assert validness
Summary:
We can use the cached current mode in one more case. Additionally make it
more explicit what a valid mode is and when to add them.
Test Plan: Relevant autotests pass. KWin Wayland nested and DRM session work.
Reviewers: KWin, davidedmundson
Reviewed By: KWin, davidedmundson
Subscribers: kde-frameworks-devel
Tags: Frameworks
Differential Revision: https://phabricator.kde.org/D23535