backends/drm: don't permanently disable VRR when the test commit fails

In 5.24, the same code path is used for testing direct scanout, so that
causes false negatives. Generally though, the user setting shouldn't be
touched, it's not really proper feedback for the driver or KWin having

(cherry picked from commit 2ef6215a)
2 jobs for Plasma/5.24 in 23 minutes and 12 seconds (queued for 4 minutes and 1 second)