drm_hwcomposer: add AutoLock to automatically handle pthread_mutex_lock

For functions that need to hold locks and also handle errors, using AutoLock
will make them much simpler and less prone to mutex handling errors.

Change-Id: Ida6545850e89186c552e3d58b2973c14852494ad
diff --git a/Android.mk b/Android.mk
index ca94f4c..f9add2f 100644
--- a/Android.mk
+++ b/Android.mk
@@ -37,6 +37,7 @@
 	system/core/libsync/include \
 
 LOCAL_SRC_FILES := \
+	autolock.cpp \
 	drmresources.cpp \
 	drmcomposition.cpp \
 	drmcompositor.cpp \