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

plugins/platforms/drm/drm_backend.cpp

@@ -455,8 +455,6 @@ void DrmBackend::updateOutputs()
 
                 if (!output->init(connector.data())) {
                     qCWarning(KWIN_DRM) << "Failed to create output for connector " << con->id();
-                    con->setOutput(nullptr);
-                    crtc->setOutput(nullptr);
                     delete output;
                     continue;
                 }