Explicitly mark DrmObject as non-copyable

It implicitly already is not copyable, due to the unique_ptr member.
However, Qt6 moc code isn't realizing that and fails to compile the
DrmPlane Q_GADGET sub-class of this.

src/backends/drm/drm_object.h

@@ -29,6 +29,7 @@ class DrmObject
 {
 public:
     virtual ~DrmObject() = default;
+    DrmObject(const DrmObject &) = delete;
 
     /**
      * Must be called to query necessary data directly after creation.