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
diff --git a/libhwcomposer/hwc_ad.cpp b/libhwcomposer/hwc_ad.cpp
index 34c241d..c2ea4ee 100644
--- a/libhwcomposer/hwc_ad.cpp
+++ b/libhwcomposer/hwc_ad.cpp
@@ -124,14 +124,28 @@
     return ret;
 }
 
-AssertiveDisplay::AssertiveDisplay() :mWbFd(-1), mDoable(false),
-        mFeatureEnabled(false), mDest(overlay::utils::OV_INVALID) {
+AssertiveDisplay::AssertiveDisplay(hwc_context_t *ctx) : mWbFd(-1),
+        mDoable(false), mFeatureEnabled(false),
+        mDest(overlay::utils::OV_INVALID) {
     int fd = openWbFb();
     if(fd >= 0) {
+        //Values in ad node:
         //-1 means feature is disabled on device
         // 0 means feature exists but turned off, will be turned on by hwc
         // 1 means feature is turned on by hwc
-        if(adRead() >= 0) {
+        // Plus, we do this feature only on split primary displays.
+        // Plus, we do this feature only if ro.qcom.ad=2
+
+        char property[PROPERTY_VALUE_MAX];
+        const int ENABLED = 2;
+        int val = 0;
+
+        if(property_get("ro.qcom.ad", property, "0") > 0) {
+            val = atoi(property);
+        }
+
+        if(adRead() >= 0 && isDisplaySplit(ctx, HWC_DISPLAY_PRIMARY) &&
+                val == ENABLED) {
             ALOGD_IF(DEBUG, "Assertive display feature supported");
             mFeatureEnabled = true;
         }