As per the binding that we have between the list store and the combo-box, when
the first element is added to the list-store, the combo box set this value as
the selected-index, and this leads to a call to cc_display_monitor_set_primary
which set the first-listed monitor as primary and unset the real primary monitor.
To avoid this, just ignore the binding when rebuilding the UI, since in this
phase control-center should just reflect the actual state without changing
anything.
Fixes https://gitlab.gnome.org/GNOME/gnome-control-center/issues/419
49957ef818