Two 30-inch TV monitors have been installed in the Dorothy Chandler Pavilion's upper lobby after patrons were locked out of the theater last week during the 90-minute Act I of two Music Center Opera performances of "Tristan and Isolde." Wednesday, 300 patrons arrived too late for the 6 p.m. performance--which began at 6:03 p.m.--and remained in the lobby until the end of the act, a spokesman said. When Music Center staffers were unable to calm the disgruntled lock-outs, opera officials offered refunds. Saturday night, about 150 patrons were locked out, the spokesman said. Postcards have since been mailed to ticket holders for the remaining two performances on Friday and Sunday reminding them to arrive early.