drm/i915: Do request retirement before marking engines as wedged

As we declare an engine as wedged, we mark all of its active requests as
in error. However, we don't want to mark successfully completed requests
as in error, which requires us to retire those requests first.

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/20170330145041.9005-1-chris@chris-wilson.co.uk