drm/amd/display: Skip finding free audio for unknown engine_id

[WHY]
ENGINE_ID_UNKNOWN = -1 and can not be used as an array index. Plus, it
also means it is uninitialized and does not need free audio.

[HOW]
Skip and return NULL.

This fixes 2 OVERRUN issues reported by Coverity.
Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/core/dc_resource.c

@@ -3187,6 +3187,9 @@ static struct audio *find_first_free_audio(
 {
 	int i, available_audio_count;
 
+	if (id == ENGINE_ID_UNKNOWN)
+		return NULL;
+
 	available_audio_count = pool->audio_count;
 
 	for (i = 0; i < available_audio_count; i++) {