Fix pre-selected refresh rate

Summary:
When we determine to show a refresh rate, it should rather be the
correct one. This was off by one.

Test Plan: Checked with my laptop display which used to wrongly display 47.99 Hz instead of 59.98

Reviewers: #plasma, broulik

Reviewed By: #plasma, broulik

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D14328