Dont use a separate mapped state of QWaylandSurfaceItem

Use the isMapped value of the QWaylandSurface

Change-Id: I7a44fa2d8d58f698cd6994e8bc8c8acf86a57775
Reviewed-by: Andy Nichols <andy.nichols@digia.com>
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>