overlay: Add support for pipe dumps.

Add support for pipe state dump useful in dumpsys.
Reduce debug log spam when mdp state changes.

Change-Id: Ie27ad00698085e2ffd6166ad2f7503fe16a571f8
diff --git a/liboverlay/overlay.h b/liboverlay/overlay.h
index a470f45..0f15baa 100644
--- a/liboverlay/overlay.h
+++ b/liboverlay/overlay.h
@@ -77,6 +77,10 @@
     void setExtFbNum(int fbNum);
     /* Returns framebuffer index of the current external display */
     int getExtFbNum();
+    /* Returns pipe dump. Expects a NULL terminated buffer of big enough size
+     * to populate.
+     */
+    void getDump(char *buf, size_t len);
 
 private:
     /* Ctor setup */