Ryan Garcia -- aka "King Ryan" -- was getting his pre-fight energy up backstage before the big weigh-in Friday night … with a little help from the undefeated David Benavidez . TMZ Sports obtained video of the professional boxer backstage in Las Vegas for his anticipated weigh-in ... practicing some killer moves with Benavidez's young son and showing off some quick jabs, while keeping his focus razor-sharp. Check the clip ... in true superstar fashion, Garcia and Benavidez are seen posing for a