msdk: dec: reset async depth to one

Currently we use an async depth of 4 as default (based on
recommendations
in msdk apps), which indicates how many asynchronous operations an
application performs
before the application explicitly synchronizes the result. As a result,
we
queue four frames in decoder which might not be good approach for
live streaming.

This patch reset the async-depth to 1 as default so that we do sync for
each frame we decode without queuing. Customer can play with already
exposed "async-depth" property for other use cases

https://bugzilla.gnome.org/show_bug.cgi?id=795783

sys/msdk/gstmsdkdec.c

@@ -64,7 +64,7 @@ enum
 };
 
 #define PROP_HARDWARE_DEFAULT            TRUE
-#define PROP_ASYNC_DEPTH_DEFAULT         4
+#define PROP_ASYNC_DEPTH_DEFAULT         1
 
 #define gst_msdkdec_parent_class parent_class
 G_DEFINE_TYPE (GstMsdkDec, gst_msdkdec, GST_TYPE_VIDEO_DECODER);