drm/i915/gtt: Reference mappable_end variable from pointer

Reference variable value from pointer, not assumed pointer destination.

Since:

commit c44ef60e
Author: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Date:   Thu Jun 25 18:35:05 2015 +0300

    drm/i915/gtt: Allow >= 4GB sizes for vm.

Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>