msm: mdss: support PP features with mdss power collapse enabled

Add support for post processing features when power collapse
feature is enabled. Every time MDSS enters power collapse mode,
all the post processing registers will get reset to 0, hence we
need to restore those PP registers when MDSS comes out of power
collapse mode.

Change-Id: I4abb41f17f24a12dddf50e01d8f69db19a464f3b
Signed-off-by: Ping Li <quicpingli@codeaurora.org>