drm/i915/gt: Split intel-gtt functions by arch

Some functions defined in the intel-gtt module are used in several
areas, but is only supported on x86 platforms.

By separating these calls and their static underlying functions to
another area, we are able to compile out these functions for
non-x86 builds and provide stubs for the non-x86 implementations.

In addition to the problematic calls, we are moving the gmch-related
functions to the new area.

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/20220330234809.1218210-2-casey.g.bowman@intel.com