drm/i915: Mark the hangcheck as idle when unparking the engines

As we unpark the engines and are about to begin a new cycle of activity,
mark the current status of the hangceck as idle so that we avoid
carrying over a stale timestamp/action into the next cycle.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180502220313.6459-2-chris@chris-wilson.co.uk