- Work around ACPI mode transition problem for recent NVIDIA 9400M chipset

- Work around ACPI mode transition problem for recent NVIDIA 9400M chipset
based Intel Macs.  Since r189055, these platforms started freezing when
ACPI is being initialized for unknown reason.  For these platforms, we just
use the old PAT layout.  Note this change is not enough to boot fully on
these platforms because of other problems but it makes debugging possible.
Note MacBook5,2 may be affected as well but it was not added here because
of lack of hardware to test.
- Initialize PAT MSR fully instead of reading and modifying it for safety.

Reported by:	rpaulo, hps, Eygene Ryabinkin (rea-fbsd at codelabs dot ru)
Reviewed by:	jhb