[platforms/x11/standalone] Fix stored refresh rate

Summary:
With 1a11abc8 X11 standalone plugin uses the AbstractOutput interface for
managing its outputs. In there the refresh rate is stored in 1/μs.

Test Plan: currentRefreshRate() provides correct rate.

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D23265