overlay: writeback: Do not create writeback instance on dumpsys

Do not create a writeback instance during dumpsys.
Instead use the dumpsys to return empty if no instance exists.
This fixes the issue where
a) Unnecessary instance is created.
b) Dumpsys has garbage, if a valid instance didn't exist

Change-Id: Ic8ea5ddd8d05203234da7c7dfccd2c2b497e62f7
diff --git a/liboverlay/overlayWriteback.cpp b/liboverlay/overlayWriteback.cpp
index e7d25b0..50aa337 100644
--- a/liboverlay/overlayWriteback.cpp
+++ b/liboverlay/overlayWriteback.cpp
@@ -215,13 +215,6 @@
     return mOpFmt;
 }
 
-void Writeback::getDump(char *buf, size_t len) const {
-    utils::getDump(buf, len, "WBData", mFbData);
-    char str[4] = {'\0'};
-    snprintf(str, 4, "\n");
-    strncat(buf, str, strlen(str));
-}
-
 //static
 
 Writeback *Writeback::getInstance() {
@@ -247,6 +240,17 @@
     }
 }
 
+bool Writeback::getDump(char *buf, size_t len) {
+    if(sWb) {
+        utils::getDump(buf, len, "WBData", sWb->mFbData);
+        char str[4] = {'\0'};
+        snprintf(str, 4, "\n");
+        strncat(buf, str, strlen(str));
+        return true;
+    }
+    return false;
+}
+
 Writeback *Writeback::sWb = 0;
 bool Writeback::sUsed = false;