Fix crash issue arising out of multiple hdmi connect/disconnects

Acquire mExtLock before proceeding further in
hwc_prepare_external to prevent crashes arising out
of updating ctx variables from two different
threads(composition and uevent threads)

Change-Id: I0f18c41b117f18aeb25572b53ef6eba3638bb826
diff --git a/libhwcomposer/hwc_utils.h b/libhwcomposer/hwc_utils.h
index e0fff77..6e95703 100644
--- a/libhwcomposer/hwc_utils.h
+++ b/libhwcomposer/hwc_utils.h
@@ -269,8 +269,8 @@
     bool mSecureMode;
     //Lock to prevent set from being called while blanking
     mutable Locker mBlankLock;
-    //Lock to protect set when detaching external disp
-    mutable Locker mExtSetLock;
+    //Lock to protect prepare & set when detaching external disp
+    mutable Locker mExtLock;
     //Vsync
     struct vsync_state vstate;
     //Drawing round when we use GPU