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/libhwcomposer/hwc.cpp b/libhwcomposer/hwc.cpp
index 2c255e1..e2ca2c7 100644
--- a/libhwcomposer/hwc.cpp
+++ b/libhwcomposer/hwc.cpp
@@ -771,8 +771,10 @@
     ctx->mRotMgr->getDump(ovDump, 1024);
     dumpsys_log(aBuf, ovDump);
     ovDump[0] = '\0';
-    Writeback::getInstance()->getDump(ovDump, 1024);
-    dumpsys_log(aBuf, ovDump);
+    if(Writeback::getDump(ovDump, 1024)) {
+        dumpsys_log(aBuf, ovDump);
+        ovDump[0] = '\0';
+    }
     strlcpy(buff, aBuf.string(), buff_len);
 }