drm/i915: Pretend the engine is always idle when mocking

If we have a mock engine and it has no more requests in flight, report
it as idle as there is no hardware to contradict us! Otherwise we
attempt to query the hw that doesn't exist and find that the hw hasn't
set its idle bit and we get upset.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170411234427.14841-2-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>