drm/msm/sde: Support PA dither feature

Support PA dither feature in DSPP to add more precision to the
original integral LUTV value, which improves quality for ABA
features. Change adds property for the feature and implements
AHB path with version 1.7.

Change-Id: I7d4a7816d91691af440366b156ddff941a669c59
Signed-off-by: Xu Yang <yangxu@codeaurora.org>