drm/i915: Assert that the request hasn't been retired

With all callers now not playing tricks with dropping the struct_mutex
between waiting and retiring, we can assert that the request is ready to
be retired.

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/1470388464-28458-19-git-send-email-chris@chris-wilson.co.uk