hwc: Add support for EXT_ONLY Layers

If there are any EXTERNAL_ONLY layers present in the list, mark
them as HWC_OVERLAY such that SF does not compose and in hwc_set
post that layer to Ext display, ignore other layers

Change-Id: Ic5db3bfa88295167d23d1fb2be7640c9fc772054
diff --git a/libhwcomposer/hwc_utils.h b/libhwcomposer/hwc_utils.h
index 6e95703..0e603ce 100644
--- a/libhwcomposer/hwc_utils.h
+++ b/libhwcomposer/hwc_utils.h
@@ -91,6 +91,7 @@
     //Video specific
     int yuvCount;
     int yuvIndices[MAX_NUM_LAYERS];
+    int extOnlyLayerIndex;
     bool needsAlphaScale;
     bool preMultipliedAlpha;
 };