drm/i915: replace X86_FEATURE_PAT with pat_enabled()

PAT can be disabled on boot with "nopat" in the command line. Replace
one x86-ism with another, which is slightly more correct to prepare for
supporting other architectures.

Cc: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211202003048.1015511-1-lucas.demarchi@intel.com