i915: initialize the free_list of the fencing atomic_helper

Just in case the llist model changes and NULL isn't valid
initialization.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170406232347.988-4-aarcange@redhat.com


Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>