Revert "KVM: VMX: Always honor guest PAT on CPUs that support self-snoop"

Revert "KVM: VMX: Always honor guest PAT on CPUs that support self-snoop"

This reverts commit 377b2f359d1f71c75f8cc352b5c81f2210312d83.

This caused a regression with the bochsdrm driver, which used ioremap()
instead of ioremap_wc() to map the video RAM.  After the commit, the
WB memory type is used without the IGNORE_PAT, resulting in the slower
UC memory type.  In fact, UC is slow enough to basically cause guests
to not boot... but only on new processors such as Sapphire Rapids and
Cascade Lake.  Coffee Lake for example works properly, though that might
also be an effect of being on a larger, more NUMA system.

The driver has been fixed but that does not help older guests.  Until we
figure out whether Cascade Lake and newer processors are working as
intended, revert the commit.  Long term we might add a quirk, but the
details depend on whether the processors are working as intended: for
example if they are, the quirk might reference bochs-compatible devices,
e.g. in the name and documentation, so that userspace can disable the
quirk by default and only leave it enabled if such a device is being
exposed to the guest.

If instead this is actually a bug in CLX+, then the actions we need to
take are different and depend on the actual cause of the bug.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>