msm: mdss: Revert changes for separating out PP flushes from commits

MDP flush behavior on MSM8974 needs to have single flush in given vsync
cycle, more than one flushes in same vsync cycle can cause unexpected
behavior such as blank screens, incorrect frame updates on LCD or some
visual artifacts. Currently post processing and frame updates have
independent flush mechanisms which in certain cases can cause visual
artifacts or blank screen. So revert changes to independently flush MDP
registers so that post processing piggybacks on flushes for frame updates.

CRs-Fixed: 494856
Change-Id: Iaa0f6f123487f57abb1b56f7d0946fa3abfe3058
Signed-off-by: Ping Li <quicpingli@codeaurora.org>