drm/display/dp_mst: drop has_audio from struct drm_dp_mst_port

Caching the has_audio in struct drm_dp_mst_port seems odd, and oddly
placed. Defer audio handling to drivers, and use the info from the
connector display info. i915 was the only one using it anyway.

Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/7d9eefdf150870479c5797f027d4c2b0a19ff583.1685437500.git.jani.nikula@intel.com