drm/i915: Assert that we don't submit to execlists whilst a preempt is pending

To complement the check in execlists_elsp_ready(), also assert that we
don't submit the same context while it has a lite restore still pending.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170124110009.28947-1-chris@chris-wilson.co.uk