drm/i915/execlists: Add a couple more validity checks to assert_pending()

Check the pending request submission is valid: that it at least has a
reference for the submission and that the request is on the active list.

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/20191203152631.3107653-1-chris@chris-wilson.co.uk