avutil: Add av_get_default_channel_layout()

Also, use the new function in the AC-3 encoder.

libavutil/audioconvert.h

@@ -124,6 +124,11 @@ void av_get_channel_layout_string(char *buf, int buf_size, int nb_channels, uint
  */
 int av_get_channel_layout_nb_channels(uint64_t channel_layout);
 
+/**
+ * Return default channel layout for a given number of channels.
+ */
+uint64_t av_get_default_channel_layout(int nb_channels);
+
 /**
  * @}
  */