ARM: dts: msm: update mdp outstanding buffer size for msm8994

Update the prefill buffer size according to hardware
recommended settings for 8994. Fix the label for
the second DSI controller.

Change-Id: I2c78b038e8e5729975b8272eef5ed91039013fac
Signed-off-by: Chandan Uddaraju <chandanu@codeaurora.org>