Avoid hypothetical crash if output->init fails

Summary:
con->setOutput and crtc->setOutput is called in Output::~Output so this
is doing it twice redundantly.

More importantly Output::~Output calls crtc->blank(), which means we
need a valid CRTC object there.

Test Plan: Compiles

Reviewers: Plasma, graesslin

Reviewed By: Plasma, graesslin

Subscribers: bshah, plasma-devel, kwin, KWin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D8779