drm/msm/sde: perform final commit on wb disable

This patch adds one additional flush/start to the writeback's
disable callflow. This allows the hardware to flush out any
necessary configuration clean up in preparation for future
commits.

CRs-Fixed: 2019295
Change-Id: Ia4090be49dd1f7689a973dd21732b429b299f11e
Signed-off-by: Clarence Ip <cip@codeaurora.org>