drm_hwcomposer: Improve handling of blending mode

1. Convert to DrmHwcBlending inside HWC hook.
2. Use map for enum_values for cleaner atomic commit preparation.

Signed-off-by: Roman Stratiienko <roman.o.stratiienko@globallogic.com>
diff --git a/drm/DrmPlane.h b/drm/DrmPlane.h
index ee619b8..5782532 100644
--- a/drm/DrmPlane.h
+++ b/drm/DrmPlane.h
@@ -81,6 +81,7 @@
   DrmProperty color_encoding_propery_;
   DrmProperty color_range_property_;
 
+  std::map<DrmHwcBlending, uint64_t> blending_enum_map_;
   std::map<DrmHwcColorSpace, uint64_t> color_encoding_enum_map_;
   std::map<DrmHwcSampleRange, uint64_t> color_range_enum_map_;
 };