drm/i915: Assert that we do not try to unsubmit a completed request

Assert that we do not try to unsubmit a completed request, as should we
try to resubmit it later, the ring is already past the request's
breadcrumb and the breadcrumb will not be updated.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180129094912.14428-1-chris@chris-wilson.co.uk
Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>