drm/i915: Assert that we do create the deferred context

In order to convince static analyzers that the allocation function
returns an error or sets ce->state, assert that it is set afterwards.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170105153023.30575-2-chris@chris-wilson.co.uk