[platforms/drm] Forward the mode changes to the OutputInterface

Summary:
While KWin gained support for changing modes it did not yet forward it
to the Wayland OutputInterface. This change implements this missing todo
by finding the matching mode and marking it as current.

Test Plan: Run kinfocenter to verify the current mode is properly set.

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D8578