drm/i915: Assert we do not try to wait on an invalid seqno

We should never insert the invalid seqno into the wait tree, so assert
we do not.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180102192500.20364-1-chris@chris-wilson.co.uk