nb/haswell: lock policy regs when disabling IOMMU

Angel Pons told me I should do it. See comments here:
https://review.coreboot.org/c/coreboot/+/81016

I see no harm in complying with the request. I'll merge
this into the main patch at a later date and try to
get this upstreamed.

Just a reminder: on Optiplex 9020 variants, Xorg locks up
under Linux when tested with a graphics card; disabling
IOMMU works around the issue. Intel graphics work just fine
with IOMMU turned on. Libreboot disables IOMMU by default,
on the 9020, so that users can install graphics cards easily.

I'm pretty sure this is the correct way to do it. The machine
still seems to boot, in this configuration.

Signed-off-by: Leah Rowe <leah@libreboot.org>

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