drm/i915: Always retire residual requests before suspend

If the driver is wedged, we skip idling the GPU. However, we may still
have a few requests still not retired following the wedging (since they
will be waiting for a background worker trying to acquire struct_mutex).
As we hold the struct_mutex, always do a quick request retirement in
order to flush the wedged path.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107257
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180717084121.28185-1-chris@chris-wilson.co.uk