drm/i915: Expose the force_audio property with DP MST

We already expose the force_audio property with SST. Do the same
with MST.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190326142556.21176-2-ville.syrjala@linux.intel.com