E. T. is finally coming home--to America's homes, that is, on videocassette. More than 5 years after its initial theatrical release, "E.T.: The Extra-Terrestrial"--the highest grossing picture of all time--is getting its home-video release this fall, it was announced Monday by MCA Home Video. MCA Home Entertainment Group President Gene Giaquinto said the video release would be "the biggest we've ever attempted," with a multimillion-dollar promotion campaign to prime the retailers and consumers. No precise figures on price or the number of units to be initially shipped were available, MCA officials said. "E. T." has grossed more than $700 million in theaters since it opened on June 11, 1982.