Davis isn't on anything, the power was always there. He's hitting a lot more fly balls so naturally he's hit more home runs while also walking more and striking out less. He's also made minor adjustments to his swing to eliminate a slight uppercut.
Davis in 2011:
You can see his front shoulder lower and dip a little as he swings.
Davis in 2013:
Here you see a noticeable difference as his bat travels through the strikezone on a much more level plane.
This change hasn't only resulted in more power but better contact in general.
His hitting profile has pretty much remained the same as last year except for three areas - the upper left, lower left, and lower right of the zone. Those areas are generally the most difficult for left-handed hitters to make contact; the difficulty is exacerbated with an uppercut swing. He's focused more on pitches in his wheelhouse and started laying off "junk".