drm/amd/display/dc/dce/dce_audio: Make function invoked by reference static

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/amd/amdgpu/../display/dc/dce/dce_audio.c:871:6: warning: no previous prototype for ‘dce60_aud_wall_dto_setup’ [-Wmissing-prototypes]

Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Leo Li <sunpeng.li@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Mauro Rossi <issor.oruam@gmail.com>
Cc: Charlene Liu <Charlene.Liu@amd.com>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dce/dce_audio.c

@@ -868,7 +868,7 @@ void dce_aud_wall_dto_setup(
 }
 
 #if defined(CONFIG_DRM_AMD_DC_SI)
-void dce60_aud_wall_dto_setup(
+static void dce60_aud_wall_dto_setup(
 	struct audio *audio,
 	enum signal_type signal,
 	const struct audio_crtc_info *crtc_info,