[drm] Make DrmOutput a QObject

Needs to emit signals when DPMS changes and also needs private slots
when DPMS changes.

backends/drm/drm_backend.cpp

@@ -502,7 +502,8 @@ void DrmBackend::bufferDestroyed(DrmBuffer *b)
 }
 
 DrmOutput::DrmOutput(DrmBackend *backend)
-    : m_backend(backend)
+    : QObject()
+    , m_backend(backend)
 {
 }
 

backends/drm/drm_backend.h

@@ -119,8 +119,9 @@ private:
     QVector<DrmBuffer*> m_buffers;
 };
 
-class DrmOutput
+class DrmOutput : public QObject
 {
+    Q_OBJECT
 public:
     struct Edid {
         QByteArray eisaId;