drm/i915/selftests: Add intel_gt_driver_late_release for mock device

Having called intel_gt_init_early() to setup the mock intel_gt, we need
to call the corresponding intel_gt_driver_late_release() to clean up.

References: dea397e8 ("drm/i915/gt: Flush retire.work timer object on unload")
References: 24635c51

 ("drm/i915: Move intel_gt initialization to a separate file")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191118094342.2193485-1-chris@chris-wilson.co.uk