x86: mtrr: Do not clear the unused ones in mtrr_commit()

Current mtrr_commit() logic assumes that MTRR MSRs are programmed
consecutively from index 0 to its maximum number, and whenever it
detects an unused one, it clears all other MTRRs starting from that
one. However this may not always be the case.

In fact, the clear is not much helpful because these MTRRs come out
of reset as disabled already. Drop the clear codes.

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>