drm/i915/execlists: Check the sentinel is alone in the ELSP

We only use sentinel requests for "preempt-to-idle" passes, so assert
that they are the only request in a new submission.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200302085812.4172450-12-chris@chris-wilson.co.uk