In the opening scene of Alejandro González Iñárritu’s unhinged and passionate tour de force “Birdman,” we see Michael Keaton, wearing nothing but a pair of tighty-whitey briefs, sitting in the lotus position and levitating four feet above the floor of his dressing room. It’s not the last time we’ll see Keaton, playing a fallen Hollywood star named Riggan Thompson who is trying to make a comeback in an ambitious Broadway play, in his underwear.