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/include/drmhwcomposer.h b/include/drmhwcomposer.h
index 8abf840..01954a1 100644
--- a/include/drmhwcomposer.h
+++ b/include/drmhwcomposer.h
@@ -55,9 +55,9 @@
 };
 
 enum class DrmHwcBlending : int32_t {
-  kNone = HWC_BLENDING_NONE,
-  kPreMult = HWC_BLENDING_PREMULT,
-  kCoverage = HWC_BLENDING_COVERAGE,
+  kNone,
+  kPreMult,
+  kCoverage,
 };
 
 struct DrmHwcLayer {