waylandsink: Fix for missing initial configure

waylandsink: Fix for missing initial configure

We were doing our initial "empty" commit on the subsurface instead of the
toplevel surface. As an incidence, we should not have received a configure
event ever, not just on mutter. This fixes the following warning when using
mutter compositor (aka gnome-shell):

  waylandsink wlwindow.c:304:gst_wl_window_new_toplevel: The compositor did not send configure event.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/2299>