Init dvr_start_address=0 and dvr_nof_data=0. to avoid 'X' after power up.

libraries/io/ddr/tb/vhdl/tb_io_ddr.vhd

@@ -140,13 +140,15 @@ BEGIN
 
   p_stimuli : PROCESS
   BEGIN
-    tb_end          <= '0';
-    dvr_en          <= '0';
-    dvr_wr_flush_en <= '0';
-    dvr_wr_not_rd   <= '0';
-    src_diag_en     <= '0';
-    snk_diag_en     <= '0';
-    expected_cnt    <= 0;
+    tb_end            <= '0';
+    dvr_en            <= '0';
+    dvr_wr_flush_en   <= '0';
+    dvr_wr_not_rd     <= '0';
+    dvr_start_address <= (OTHERS=>'0');
+    dvr_nof_data      <= (OTHERS=>'0');
+    src_diag_en       <= '0';
+    snk_diag_en       <= '0';
+    expected_cnt      <= 0;
 
     proc_common_wait_until_high(dvr_clk, dvr_done);