x86/mtrr: Revert 90b926e6 ("x86/pat: Fix pat_x_mtrr_type() for MTRR disabled case")

Commit

  90b926e6 ("x86/pat: Fix pat_x_mtrr_type() for MTRR disabled case")

broke the use case of running Xen dom0 kernels on machines with an
external disk enclosure attached via USB, see Link tag.

What this commit was originally fixing - SEV-SNP guests on Hyper-V - is
a more specialized situation which has other issues at the moment anyway
so reverting this now and addressing the issue properly later is the
prudent thing to do.

So revert it in time for the 6.2 proper release.

  [ bp: Rewrite commit message. ]

Reported-by: Christian Kujau <lists@nerdbynature.de>
Tested-by: Christian Kujau <lists@nerdbynature.de>
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/4fe9541e-4d4c-2b2a-f8c8-2d34a7284930@nerdbynature.de