hal: enable audio over display port

Add support for audio over display port.

Change-Id: I3480d89c61ee5e5983715ec943fb02b1a9d5d151
diff --git a/hal/platform_api.h b/hal/platform_api.h
index ec64206..25ff64c 100644
--- a/hal/platform_api.h
+++ b/hal/platform_api.h
@@ -158,4 +158,5 @@
 bool platform_check_codec_dsd_support(void *platform);
 bool platform_check_codec_asrc_support(void *platform);
 int platform_get_backend_index(snd_device_t snd_device);
+int platform_get_ext_disp_type(void *platform);
 #endif // AUDIO_PLATFORM_API_H