vnc: move check into vnc_cursor_define

Move the check whenever a cursor exists into the vnc_cursor_define()
function so callers don't have to do it.

Suggested-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-id: 20210112134120.2031837-2-kraxel@redhat.com