hwcomposer: Add adb property to disable Mixed Mode

This change adds adb property support to disable mixed mode in
MDP Comp. Will be helpful in debugging issues to narrow down the
root cause.

Change-Id: I49b737bb8755dc3a35db2e1ea8ccb2043135d2b0
diff --git a/libhwcomposer/hwc_mdpcomp.h b/libhwcomposer/hwc_mdpcomp.h
index e2800d2..6e04918 100644
--- a/libhwcomposer/hwc_mdpcomp.h
+++ b/libhwcomposer/hwc_mdpcomp.h
@@ -166,6 +166,7 @@
 
     int mDpy;
     static bool sEnabled;
+    static bool sEnableMixedMode;
     static bool sDebugLogs;
     static bool sIdleFallBack;
     static int sMaxPipesPerMixer;