ARM: dts: msm: Update MDP writeback QoS configuration

Right now, writeback QoS registers are being programmed dynamically
when respective paths are being exercised. Starting 8916, wb QoS
is starting to differ from other chip-sets and since these are
chip-set specific settings, hence configure them using device-tree.

Change-Id: I7eb3717e3756f1eb09c0f1e972fc326854b8ee37
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>