drm/i915: Markup potential lock for i915_active

Make the lockchains more deterministic via i915_active by flagging the
potential lock.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190703091726.11690-7-chris@chris-wilson.co.uk