radeon: don't emit streamout state if there are no streamout buffers

This could happen if set_stream_output_targets is called twice
in a row without a draw call in between.

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>

src/gallium/drivers/radeon/r600_streamout.c

@@ -137,6 +137,8 @@ void r600_set_streamout_targets(struct pipe_context *ctx,
 
 	if (num_targets) {
 		r600_streamout_buffers_dirty(rctx);
+	} else {
+		rctx->streamout.begin_atom.dirty = false;
 	}
 }