x86/PAT: Have pat_enabled() properly reflect state when running on Xen

After commit ID in the Fixes: tag, pat_enabled() returns false (because
of PAT initialization being suppressed in the absence of MTRRs being
announced to be available).

This has become a problem: the i915 driver now fails to initialize when
running PV on Xen (i915_gem_object_pin_map() is where I located the
induced failure), and its error handling is flaky enough to (at least
sometimes) result in a hung system.

Yet even beyond that problem the keying of the use of WC mappings to
pat_enabled() (see arch_can_pci_mmap_wc()) means that in particular
graphics frame buffer accesses would have been quite a bit less optimal
than possible.

Arrange for the function to return true in such environments, without
undermining the rest of PAT MSR management logic considering PAT to be
disabled: specifically, no writes to the PAT MSR should occur.

For the new boolean to live in .init.data, init_cache_modes() also needs
moving to .init.text (where it could/should have lived already before).

  [ bp: This is the "small fix" variant for stable. It'll get replaced
    with a proper PAT and MTRR detection split upstream but that is too
    involved for a stable backport.
    - additional touchups to commit msg. Use cpu_feature_enabled(). ]

Fixes: bdd8b6c98239 ("drm/i915: replace X86_FEATURE_PAT with pat_enabled()")
Signed-off-by: Jan Beulich <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Acked-by: Ingo Molnar <[email protected]>
Cc: <[email protected]>
Cc: Juergen Gross <[email protected]>
Cc: Lucas De Marchi <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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