Get name and refresh rate from output device

Summary:
We can simplify the AbstractWaylandOutput code some more by getting name and
refresh rate of an output from the always available output device object.

Test Plan: Tested with DRM, Wayland nested and virtual backends.

Reviewers: KWin

Subscribers: davidedmundson, apol, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D23546