Ensure that the model is set when resetResizing is called

Previously, KDirOperatorDetailView::setModel called resetResizing before
QTreeView::setModel was called. Thus, when resetResizing called model()
to obtain the model, it was still nullptr, leading to the following
warning: "QObject::connect: invalid null parameter"

As nothing we do in KDirOperatorDetailView::setModel depends on the
model not being set yet, we simply move the call to
KDirOperatorDetailView::setModel to the beginning of the method, to
ensure that model() returns the new model