ARM: dts: msm: increase mdp dma priority to 3

Increase mdp dma bus master priority to 3 from 2.
This change is needed to avoid underuns in certain
usecases where many clients are consuming bandwidth.

Change-Id: I194b4c8195cd176bb9b66ccf2c7e766ea9620dfb
Signed-off-by: Xiaoming Zhou <zhoux@codeaurora.org>