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>
diff --git a/arch/x86/cpu/mtrr.c b/arch/x86/cpu/mtrr.c
index 73cf7bb..14c644e 100644
--- a/arch/x86/cpu/mtrr.c
+++ b/arch/x86/cpu/mtrr.c
@@ -155,7 +155,7 @@
 	debug("open done\n");
 	qsort(req, gd->arch.mtrr_req_count, sizeof(*req), h_comp_mtrr);
 	for (i = 0; i < gd->arch.mtrr_req_count; i++, req++)
-		set_var_mtrr(i, req->type, req->start, req->size);
+		mtrr_set_next_var(req->type, req->start, req->size);
 
 	debug("close\n");
 	mtrr_close(&state, do_caches);