msm: mdss: ensure proper state transition during ULPS exit

During stress environment, sometimes DSI ULPS exit sequence
does not work properly. Add memory barriers while programing
DSI lane register to ensure proper state transition of DSI
lane during ULPS exit.

Change-Id: Ib50958cf342bf23ab44f5a9d3bdf60adad824145
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>