drm_hwcomposer: Simplify LayerTransform

Fixes clang-analyzer-optin.core.EnumCastOutOfRange clang tidy check.

Change-Id: I0a88d1ef084848c924198e8bd3831533b6578675
Signed-off-by: Roman Stratiienko <r.stratiienko@gmail.com>
diff --git a/drm/DrmProperty.h b/drm/DrmProperty.h
index 2683ad8..860f9ca 100644
--- a/drm/DrmProperty.h
+++ b/drm/DrmProperty.h
@@ -68,6 +68,8 @@
   auto AddEnumToMapReverse(const std::string &name, E value,
                            std::map<uint64_t, E> &map) -> bool;
 
+  auto GetEnumMask(uint64_t &mask) -> bool;
+
   explicit operator bool() const {
     return id_ != 0;
   }