Pianist Van Cliburn is slowly returning to the public spotlight after breaking a long sabbatical. After performing at the White House during the summit meeting in December, Cliburn again appeared before the Reagans in February at the inauguration of the Bob Hope Cultural Center in Palm Springs. NBC will broadcast the performance March 5. So far the pianist has accepted only one other offer--conductor Zubin Mehta's invitation to accompany the New York Philharmonic on its tour to the Soviet Union in May and June. Cliburn's big break came in Moscow 30 years ago, when he won the Tchaikovsky International Competition at the age of 24.