x86/speculation/mds: fix microcode late loading

x86/speculation/mds: fix microcode late loading

In the microcode late loading case we have to:
- clear the CPU bugs related to MDS to be re-evaluated
- add proper evaluation of the MDS state and enable mitigation if necessary.

If the user has enforced off or idle mitigation, we keep it. Also if the
microcode fixes the MDS bug, mitigation will be turned off.

Orabug: 29526900
CVE: CVE-2018-12126
CVE: CVE-2018-12130
CVE: CVE-2018-12127

Signed-off-by: Mihai Carabas <mihai.carabas@oracle.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>