drm/i915: Don't add the "force audio" property to DP connectors that don't support audio

No point in adding the "force audio" property to DP connectors
that don't support audio (g4x or port A).

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171129164303.18793-2-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>