Trigger manually map update after initialization

If all of the tiles were loaded from cache and plugin for the map was set after creation of the declarative map, it can occur that item is not updated

Task-number: QTBUG-60021
Change-Id: I260b6219b65ef16726aa767d3a83c6fc756642b5
Reviewed-by: Paolo Angelelli <paolo.angelelli@qt.io>