drm/i915/gtt: Skip clearing the GGTT under gen6+ full-ppgtt

If we know that the user cannot access the GGTT, by virtue of having a
segregated memory area, we can skip clearing the unused entries as they
cannot be accessed.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Reviewed-by: Matthew Auld <matthew.william.auld@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190419201207.5477-1-chris@chris-wilson.co.uk