drm/i915: Serialise i915_active_wait() with its retirement

drm/i915: Serialise i915_active_wait() with its retirement

As the i915_active.retire() may be running on another CPU as we detect
that the i915_active is idle, we may not wait for the retirement itself.
Wait for the remote callback by waiting for the retirement worker.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=112424
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/20191202140133.2444217-2-chris@chris-wilson.co.uk