overview: Avoid entering in overview mode during display grab op

ClutterGrab and display grab operations are pretty much mutually
exclusive, but there's no mechanism to ensure a ClutterGrab does
not step on top of an existing grab operation.

This looks the only place that could trigger that (e.g. start
dragging window, then move pointer to top/left corner), so skip
over the situation here explicitly.

Fixes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/5397
Fixes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/5181
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2281>