msm: mdss: Support demo mode for post processing

Add demo mode feature for numerous post processing features. In dual pipe
case, demo mode allows for either the left or right pipe's feature to be
disabled to showcase its effects. Features supported include assertive
display, gamut mapping, panel gamma correction, inverse gamma correction,
dithering, picture adjustment (v2 only), and polynomial color correction.

Change-Id: I8bceeffc11967ff23497df7739e6f99c30d2b10f
Signed-off-by: Carl Vanderlip <carlv@codeaurora.org>
Signed-off-by: Ping Li <quicpingli@codeaurora.org>