[Cuttlefish] Replace deprectated method KDeclarative::setupBindings()

Summary:
Replace method call like described in the documentation. Also we don't need to
cast &engine because a QQmlApplicationEngine is a QQmlEngine.

Test Plan: compiles

Reviewers: #plasma, apol

Reviewed By: apol

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D25842