drm: radeon: replace 0 with NULL

Replace 0 with NULL to fix sparse tool  warning
 warning: Using plain integer as NULL pointer
Acked-by: NChristian König <christian.koenig@amd.com>
Reviewed-by: NZhan Liu <zhan.liu@amd.com>
Signed-off-by: NJules Irenge <jbi.octave@gmail.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/radeon_audio.c

@@ -288,7 +288,7 @@ static void radeon_audio_interface_init(struct radeon_device *rdev)
 	} else {
 		rdev->audio.funcs = &r600_funcs;
 		rdev->audio.hdmi_funcs = &r600_hdmi_funcs;
-		rdev->audio.dp_funcs = 0;
+		rdev->audio.dp_funcs = NULL;
 	}
 }