drm/i915: Share the common force-audio property between connectors

Make the audio property creation routine common and share the single
property between the connectors.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Keith Packard <keithp@keithp.com>