ui: fix dcl unregister

register checks for dcl->ds being NULL, to avoid registering
the same dcl twice.

Therefore dcl->ds must be cleared on unregister, otherwise
un-registering and re-registering doesn't work.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1510809
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20171109105154.29414-1-kraxel@redhat.com
