drm/i915: Report engines are idle if already parked

If we have parked, then we must have passed an idleness test and still
be idle. We chose not to use this shortcut in the past so that we could
use the idleness test at any time and inspect HW. However, some HW like
Sandybridge, doesn't like being woken up frivolously, so avoid doing so.

References: 0b702dca ("drm/i915: Avoid waking the engines just to check if they are idle")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190227214159.7946-1-chris@chris-wilson.co.uk