[server] Cache current mode

Summary:
Instead of looping over all modes for retrieving current mode data copy it
to a separate variable.

Test Plan:
KWin auto tests pass and KScreen in Wayland nested session shows correct
information.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, kde-frameworks-devel

Tags: #frameworks

Maniphest Tasks: T11459

Differential Revision: https://phabricator.kde.org/D23493