drm/i915: Drain any freed objects prior to hibernation

drm/i915: Drain any freed objects prior to hibernation

As we call into the shrinker during freeze, we may have freed more
objects since we idled during i915_gem_suspend. Make sure we flush the
i915_gem_free_objects worker prior to saving the unwanted pages into the
hibernation image.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170407102552.5781-2-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>