drm_hwcomposer: Modify source_layers_ to be integer instead of array

It isn't used as an array. Simplify the logic.

Signed-off-by: Roman Stratiienko <roman.o.stratiienko@globallogic.com>
Tested-by: Martin Juecker <martin.juecker@gmail.com>
Reviewed-by: Matvii Zorin <matvii.zorin@globallogic.com>
diff --git a/compositor/DrmDisplayComposition.h b/compositor/DrmDisplayComposition.h
index 2ddfaac..cbc0a90 100644
--- a/compositor/DrmDisplayComposition.h
+++ b/compositor/DrmDisplayComposition.h
@@ -32,35 +32,28 @@
 class Importer;
 class Planner;
 
+constexpr size_t kUndefinedSourceLayer = UINT16_MAX;
+
 class DrmCompositionPlane {
  public:
   DrmCompositionPlane() = default;
   DrmCompositionPlane(DrmCompositionPlane &&rhs) = default;
   DrmCompositionPlane &operator=(DrmCompositionPlane &&other) = default;
-  DrmCompositionPlane(DrmPlane *plane) : plane_(plane) {
-  }
   DrmCompositionPlane(DrmPlane *plane, size_t source_layer)
-      : plane_(plane), source_layers_(1, source_layer) {
+      : plane_(plane), source_layer_(source_layer) {
   }
 
   DrmPlane *plane() const {
     return plane_;
   }
-  void set_plane(DrmPlane *plane) {
-    plane_ = plane;
-  }
 
-  std::vector<size_t> &source_layers() {
-    return source_layers_;
-  }
-
-  const std::vector<size_t> &source_layers() const {
-    return source_layers_;
+  size_t source_layer() const {
+    return source_layer_;
   }
 
  private:
   DrmPlane *plane_ = NULL;
-  std::vector<size_t> source_layers_;
+  size_t source_layer_ = kUndefinedSourceLayer;
 };
 
 class DrmDisplayComposition {