hwc: Writeback for ad is needed only for split displays.

Writeback for assertive display feature is needed only for
split displays and only if ro.qcom.ad is set to 2 by OEM.
This patch removes the unnecessary code and adds the relevant
conditional checks for split displays and property value

Change-Id: Ic36f28441d1a6e93132316b33fe1cf98a6e1efe4
4 files changed