sdm: Report mixer configuration in dumpsys

Report the maximum mixer stages for each display.

Change-Id: Ibdf1863b0ff6782860117e2467ea637c8b419d9b

sdm/include/utils/debug.h

@@ -65,6 +65,7 @@ class Debug {
   static bool IsRotatorDownScaleDisabled();
   static bool IsDecimationDisabled();
   static bool IsPartialUpdateEnabled();
+  static int GetMaxPipesPerMixer();
 
  private:
   Debug();