drm/i915/gt: Treat idling as a RPS downclock event

If we park/unpark faster than we can respond to RPS events, we never
will process a downclock event after expiring a waitboost, and thus we
will forever restart the GPU at max clocks even if the workload switches
and doesn't justify full power.

Closes: https://gitlab.freedesktop.org/drm/intel/issues/1500
Fixes: 3e7abf81

 ("drm/i915: Extract GT render power state management")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Andi Shyti <andi.shyti@intel.com>
Cc: Lyude Paul <lyude@redhat.com>
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200322163225.28791-1-chris@chris-wilson.co.uk
Cc: <stable@vger.kernel.org> # v5.5+