drm_hwcomposer: get gralloc buffer usage during hwc_set

This is needed during the layer composition planning phase to determine if a
layer is protected and therefore needs to be in its own overlay.

BUG=chrome-os-partner:43674

Change-Id: I33510a5a56b57aaaf55732149ae3e3d2f92ada79
diff --git a/drm_hwcomposer.h b/drm_hwcomposer.h
index 5a45782..4a8b4a3 100644
--- a/drm_hwcomposer.h
+++ b/drm_hwcomposer.h
@@ -208,6 +208,7 @@
 
 struct DrmHwcLayer {
   buffer_handle_t sf_handle = NULL;
+  int gralloc_buffer_usage = 0;
   DrmHwcBuffer buffer;
   DrmHwcNativeHandle handle;
   DrmHwcTransform transform = DrmHwcTransform::kIdentity;