hal: enable audio over display port

Add support for audio over display port.

Change-Id: I3480d89c61ee5e5983715ec943fb02b1a9d5d151
diff --git a/hal/audio_hw.h b/hal/audio_hw.h
index 664d1fc..c1797bd 100644
--- a/hal/audio_hw.h
+++ b/hal/audio_hw.h
@@ -146,6 +146,8 @@
     USECASE_AUDIO_PLAYBACK_AFE_PROXY,
     USECASE_AUDIO_RECORD_AFE_PROXY,
 
+    USECASE_AUDIO_PLAYBACK_EXT_DISP_SILENCE,
+
     AUDIO_USECASE_MAX
 };