Fix QQmlDelegateModel ignoring layoutChange in certain situations

Fix a regression introduced by commit a0aefe1 which caused the model to
ignore layout changes if d->m_adaptorModel.rootIndex was just a descendant
of any of the parent indexes, or when no parent indexes at all were provided
in the notification.

Task-number: QTBUG-39492
Change-Id: I4c97929d25ef75947ccfcbbe5bc234096689c58d
Reviewed-by: Alan Alpert <aalpert@blackberry.com>