Act or Peter Lawford's condition remains the same. A spokesman at Cedars-Sinai Medical Center in Los Angeles reports Lawford is in a coma and in critical condition. His wife and children have been at his bedside, and his old friend Elizabeth Taylor has visited several times . . . Former Arena Stage actor Robert Prosky, who plays the roll-call sergeant on "Hill Street Blues," says he was offered the role of coach on "Cheers" but turned it down. "They had a commitment for 13 shows and they wanted me to sign a seven-year contract. I knew their show was going to succeed and that's why I turned it down," Prosky says in the upcoming TV Guide . . . Things can't be all that bad for former auto executive John DeLorean. Even though he advertised for $1 million to help pay off his legal debt in the federal drug case he won in August, DeLorean and his two children are vacationing in a $275-a-night suite in Vail, Colo. . . . President Reagan and Nancy Reagan will continue their California Christmas tradition by celebrating tonight with U.S. Information Agency director Charles Z. Wick and his wife, Mary Jane. Wick, incidentally, surprised his wife last week with an early Christmas present -- a 10-carat diamond ring. . .