x86: mtrr: Skip MSRs that were already programmed in mtrr_commit()

At present mtrr_commit() programs the MTRR MSRs starting from
index 0, which may overwrite MSRs that were already programmed
by previous boot stage or FSP.

Switch to call mtrr_set_next_var() instead.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested on chromebook_coral, chromebook_samus, chromebook_link, minnowmax
Tested-by: Simon Glass <sjg@chromium.org>