Although you are legally allowed to come back to Qatar and work, you are still required to take a formal NOC from your last employer. If your visa was cancelled by your then employer under normal circumstances, you must not have any problem getting their go ahead nod with a signature and they are obliged to sign your NOC. Only in situations where you made a promise of coming back but did not turn up, you ex-employer might have some dues to settle with you for example, that you might have a problem.