drm/i915/dp: stop caching has_audio in struct intel_dp

Use the information stored in display info.

Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/695bbe8b5ebee6e492f95a1c818da15691752dc8.1685437500.git.jani.nikula@intel.com