Simplify XRandR::outputChanged

By returning early, we can avoid some of the nesting. The old code would
call m_configChangeCompressor->start() in any case, move that to the
beginning and then use early returns.
There is actually very little code changed, "git show -w" shows the
actual changes. The fallthrough was hard to read.

Reviewers: #plasma

Subscribers: mart, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D14764