CHRIS JANSON is back on the bottle . . . of Mountain Dew. You may have heard that he had a serious addiction to the stuff . . . and went cold turkey for two years. But he had a hit song in August, and it was time to celebrate. Quote, "I hadn't drunk Mountain Dew in two years until 'Done' hit Number One for the first week, and then I was like, 'Well, this is a great reason to celebrate. And by the time it was a two-week Number One, I had drunk well over two 12-packs."