drm_hwcomposer: set the composition map's display field

Change-Id: I67842cdfd0afab67a81448c1b64da47d2f933db3
diff --git a/hwcomposer.cpp b/hwcomposer.cpp
index 0a7be52..6a0f4cd 100644
--- a/hwcomposer.cpp
+++ b/hwcomposer.cpp
@@ -501,6 +501,7 @@
 
     layers_map.emplace_back();
     DrmCompositionDisplayLayersMap &map = layers_map.back();
+    map.display = i;
     std::vector<size_t> &indices_to_composite = layers_indices[i];
     for (size_t j : indices_to_composite) {
       hwc_layer_1_t *sf_layer = &dc->hwLayers[j];