drm/i915: Assert no external observers when unwind a failed request alloc

Before we return the request back to the kmem_cache after a failed
i915_gem_request_alloc(), we should assert that it has not been added to
any global state tracking.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161125131718.20978-2-chris@chris-wilson.co.uk