drm_hwcomposer: Clean up SetLayerBlendMode for hwc3

Don't use the existing HWC2 api. Remove related hwc2/hwc3 conversion
helper.

Change-Id: Ie3e53d1854df2e895360bd0c445d398649ab310a
Signed-off-by: Drew Davenport <ddavenport@google.com>
diff --git a/hwc2_device/HwcDisplay.h b/hwc2_device/HwcDisplay.h
index 87d2da7..6bc3b6b 100644
--- a/hwc2_device/HwcDisplay.h
+++ b/hwc2_device/HwcDisplay.h
@@ -58,7 +58,7 @@
     return configs_;
   }
 
-  // HWC Hooks
+  // HWC2 Hooks - these should not be used outside of the hwc2 device.
   HWC2::Error AcceptDisplayChanges();
   HWC2::Error CreateLayer(hwc2_layer_t *layer);
   HWC2::Error DestroyLayer(hwc2_layer_t layer);