drm/i915/gt: Pull restoration of GGTT fences underneath the GT

Make the GT responsible for restoring its fence when it wakes up from
suspend.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200316113846.4974-2-chris@chris-wilson.co.uk