drm/i915: Adjust the sentinel assert to match implementation

Sentinels are supposed to be last requests in the elsp queue, not the
only one, so adjust the assert accordingly.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200607222108.14401-1-chris@chris-wilson.co.uk


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