drm/i915: Add is-completed assert to request retire entrypoint

While we will check that the request is completed prior to being
retired, by placing an assert that the request is complete at the
entrypoint of the function we can more clearly document the function's
preconditions.

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/20161125131718.20978-1-chris@chris-wilson.co.uk