(Updates with markets open) DUBAI, March 30 (Reuters) - Middle East stock markets weremixed by midday on Monday, driven by weaker oil andcompany-specific news as well as stocks going ex-dividend. Oil prices extended losses from the previous session as Iranand six world powers tried to reach a deal that could add oil tothe market if sanctions against Tehran are lifted. The two sidesaim to reach a preliminary agreement by Tuesday. The main Saudi index slipped 0.2 percent aspetrochemicals giant Saudi Basic Industries Co edgeddown 0.4 percent. ...