By Tim Kenneally LOS ANGELES (TheWrap.com) – “9021″-oh, yeah! Jason Priestley is coming to “CSI.” Former “Beverly Hills 90210″ star Priestley has been booked as a guest star on the CBS crime drama for its 300th episode, which will air Oct 23. Priestley will play Mr. Witten, a reclusive casino mogul who once threw the most lavish parties in Las Vegas until he was accused of murder. Still handsome and charming, years of battling his addictions have taken their toll. The heir to a classic movie studio empire and a lover of film magic, he’s looking to believe in his dreams again. …