NBC's Sunday Night Football coverage of the Chicago Bears/Green Bay Packers game averaged 20.6 million viewers, according to Nielsen overnight ratings. Because of the live nature of The Game, that figure is almost certain to rise when final figures are released late today or tomorrow. The game overwhelmed just about everything else on television, which, on Christmas night, included mostly reruns. In fact in the 9 00 p.m. hour the game drew twice the number of viewers who tuned in to the other three major broadcast networks combined . At 10 00 p.m., with competition from only CBS and ABC, the game averaged 20.7 million viewers, while the other two networks together drew just 9.4 million. On Saturday night, Christmas Eve, NBC's annual airing of the 75-year-old movie It's A Wonderful Life turned out to be the most-watched telecast of the night. It didn't take much to win, however. with most people doing something other than watching TV on the night of Santa Claus's arrival. Just 4.6 million viewers tuned in.