drm/i915: Skip remap_io_mapping() for non-x86 platforms

Only hw that supports mappable aperture would hit this path
vm_fault_gtt/vm_fault_tmm, So we never hit this function
remap_io_mapping() in discrete, So skip this code for non-x86
architectures.

v2: use IS_ENABLED () instead of #if defined

v3: move function prototypes from i915_drv.h to i915_mm.h

v4: added kernel error message in stub function

v5: fixed compilation warnings

v6: checkpatch style

Signed-off-by: Siva Mullati <siva.mullati@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211208041215.763098-1-siva.mullati@intel.com