sf: Fix incorrect state reporting in dumpsys

Dumpsys utility accesses layer compositionType variable
with out proper protection. These variables are modified
during hwc_prepare call. Existing HAL lock protection is not
sufficient to address this issue. Failure to do this will
result in incorrect state reporting in dumpsys.

A new displayLock mutex in HWComposer will be used in both
dumpsys and draw calls to ensure correct state is accessed.

Change-Id: I8a57de59525adc0e089b3bed95c067c01e42b666
(cherry picked from commit e54506b81a3b81683056ad48294e37d6b4b4e36b)
diff --git a/services/surfaceflinger/DisplayHardware/HWComposer.h b/services/surfaceflinger/DisplayHardware/HWComposer.h
index 28d8c65..cc98b4c 100644
--- a/services/surfaceflinger/DisplayHardware/HWComposer.h
+++ b/services/surfaceflinger/DisplayHardware/HWComposer.h
@@ -354,6 +354,8 @@
     // mLists[i>0] can be NULL. that display is to be ignored
     struct hwc_display_contents_1*  mLists[MAX_HWC_DISPLAYS];
     DisplayData                     mDisplayData[MAX_HWC_DISPLAYS];
+    // protect mDisplayData from races between prepare and dump
+    mutable Mutex mDisplayLock;
     size_t                          mNumDisplays;
 
     cb_context*                     mCBContext;