drm/i915/selftests: Flush any i915_active callback work as well

Make trebly sure that all possible callbacks and their delayed brethren
are complete before asserting that the i915_active should be idle after
flushing all barriers.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191023235359.27132-1-chris@chris-wilson.co.uk