x86/mtrr: Enable MTRR's before enabling caching

Fix up the following commit by enabling the MTRR's before enabling caching.

7756fe7 x86: Minimize work done with the caches disabled in mtrr functions.

Also fix two typos in comments.

Change-Id: If751b815f9dab781fc38c898cf692f0940c57695
Signed-off-by: Isaac Christensen <isaac.christensen@se-eng.com>
Reviewed-on: http://review.coreboot.org/6969
Reviewed-by: Aaron Durbin <adurbin@google.com>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Tested-by: build bot (Jenkins)