drm/i915: Split i915_run_as_guest into x86 and non-x86

Split i915_run_as_guest() into a more arch-friendly function
as non-x86 builds do not support this functionality.

Signed-off-by: Casey Bowman <casey.g.bowman@intel.com>
Acked-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220331204343.1256150-2-casey.g.bowman@intel.com