drm/i915/gt: Flush ongoing retires during wait_for_idle

Synchronise with any background retires and parking we may have spawned,
so that all requests are accounted for.

Closes: https://gitlab.freedesktop.org/drm/intel/issues/878


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200102231604.1669010-1-chris@chris-wilson.co.uk