Fernando Mendoza and Kirk Cousins just became best friends. The two Raiders newcomers, who are vying for the role of QB1 in Las Vegas this season, each participated in a “Step Brothers” parody for the team’s schedule release video on Thursday — making a clear effort to quiet those who have predicted a failed relationship between the competitors. Mimicking the way John C. Reilly and Will Ferrell went from enemies to BFFs in the 2008 classic, Mendoza and Cousins first lined up on the Raiders’