avcodec: autodetect hardware acceleration by default

modules/codec/avcodec/avcodec.c

@@ -139,7 +139,7 @@ vlc_module_begin ()
     add_obsolete_string( "ffmpeg-codec" ) /* removed since 2.1.0 */
     add_string( "avcodec-codec", NULL, CODEC_TEXT, CODEC_LONGTEXT, true )
     add_obsolete_bool( "ffmpeg-hw" ) /* removed since 2.1.0 */
-    add_module( "avcodec-hw", "hw decoder", "none", HW_TEXT, HW_LONGTEXT, false )
+    add_module( "avcodec-hw", "hw decoder", NULL, HW_TEXT, HW_LONGTEXT, false )
 #if defined(FF_THREAD_FRAME)
     add_obsolete_integer( "ffmpeg-threads" ) /* removed since 2.1.0 */
     add_integer( "avcodec-threads", 0, THREADS_TEXT, THREADS_LONGTEXT, true );