i965: Print the type and writemask on null destinations.

These are often useful in debugging, and the writemask (actually
"Channel Enables") determines more than just what goes into the
destination.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/drivers/dri/i965/brw_disasm.c

@@ -726,7 +726,7 @@ reg(FILE *file, unsigned _reg_file, unsigned _reg_nr)
       switch (_reg_nr & 0xf0) {
       case BRW_ARF_NULL:
          string(file, "null");
-         return -1;
+         break;
       case BRW_ARF_ADDRESS:
          format(file, "a%d", _reg_nr & 0x0f);
          break;