drm/i915: Require INTEL_GTT to depend on X86

The intel-gtt module is not used on other, non-x86 platforms, so we
will restrict it to x86 platforms only.

Signed-off-by: Casey Bowman <casey.g.bowman@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/20220330234809.1218210-3-casey.g.bowman@intel.com