drm/i915/selftests: Use common mock_engine::advance

Replace the open-coding of advance with a call instead.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190121222117.23305-19-chris@chris-wilson.co.uk