Clocking in at 80 minutes, the two-episode season finale of “Extras” (“Extras: The Extra Special Series Finale”), which will air Sunday on HBO, comes close to being a standalone movie. It’s also a Christmas movie of sorts, with sentimental passages, a fleeting biblical reference or two (“No room at the inn…”), marquee names (Clive Owen, George Michael, Gordon Ramsay), and a self-loathing narcissist as its hero who gains a sneak peek at redemption at the end.