drm/i915: Exclude reserved stolen from driver use

Remove the portion of stolen memory reserved for private use from driver
access.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
cc: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211208153404.27546-2-ramalingam.c@intel.com