Avoid unnecessary work when linking observers

We already do most of the work of setFirstObserver() in the method body
before.

Change-Id: Ia31f19ca656675dddb692609d8875c5d48e967f1
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/corelib/kernel/qproperty.cpp

@@ -83,8 +83,8 @@ void QPropertyBindingDataPointer::addObserver(QPropertyObserver *observer)
         observer->next = firstObserver;
         if (observer->next)
             observer->next->prev = &observer->next;
+        d = reinterpret_cast<quintptr>(observer);
     }
-    setFirstObserver(observer);
 }
 
 /*!