loginDialog: Don't reconnect on each reset with gdmClient

loginDialog: Don't reconnect on each reset with gdmClient

When resetting the greeter proxy, it's destroyed and then recreated
again.

This destroy/recreate thing doesn't give any info to GDM and the greeter
interface destroyed has the same state as the newly created one.

Each time get_greeter_sync is called when greeter is destroyed makes GDM
create a new connection which stores in a list. This means that for each
reset a new unnecessary connection is created and stored.

GDM was leaking some objects which combined with this excessive connection
creation was causing a fd leak on each reset.

To make better use of resources and avoid problems, use ensure instead
of reset greeter proxy.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3749>