xwayland: Add xwl_cursor_clear_frame_cb()

The code to clear a cursor pending frame callback was duplicated in
multiple places in the code.

Introduce a new xwl_cursor_clear_frame_cb() function and remove the
duplicated code.

No functional change.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Carlos Garnacho <carlosg@gnome.org>
5 jobs for xwayland-cursor-cleanup in 6 minutes and 23 seconds (queued for 4 seconds)