x86: Skip setting up MTRRs in slimbootloader

The setting up MTRRs have already been done in previous
Slim Bootloader stages.

Signed-off-by: Aiden Park <aiden.park@intel.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>