drm/i915/gt: Do not use stolen on MTL

Use smem on MTL due to a HW bug in MTL that prevents
reading from stolen memory using LMEM BAR.

Cc: Oak Zeng <oak.zeng@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Andi Shyti <andi.shyti@linux.intel.com>
Cc: Andrzej Hajda <andrzej.hajda@intel.com>
Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Oak Zeng <oak.zeng@intel.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230713150142.12700-1-nirmoy.das@intel.com