intel_overlay already tracks its last flip request, along with action to
take after its completion. Refactor intel_overlay to reuse the common
i915_gem_active tracker.

v2: Now using i915_gem_retire_fn typedef

References: https://bugs.freedesktop.org/show_bug.cgi?id=93730
References: https://bugs.freedesktop.org/show_bug.cgi?id=96851Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470293567-10811-18-git-send-email-chris@chris-wilson.co.uk