Revert "os: reset Audio_out stream prior to calling start"

This reverts commit d8b95c2.

Some clients might want to fiddle with the clients play position
(e.g. the mixer) within 'Audio_out::start()' and we might reset
it to an old value if we do it afterwards.

Fixes genodelabs#1796.

repos/os/include/audio_out_session/client.h

@@ -91,10 +91,10 @@ class Audio_out::Session_client : public Genode::Rpc_client<Session>
 
 		void start()
 		{
+			call<Rpc_start>();
+
 			/* reset tail pointer */
 			stream()->reset();
-
-			call<Rpc_start>();
 		}
 
 		void stop() { call<Rpc_stop>();  }