drm/i915/audio: group audio under anonymous struct in drm_i915_private

With an anonymous struct, this can be pure hierarchical organization
without code changes. We'll follow up with adding a name to the
sub-struct separately.

Cc: Dave Airlie <airlied@redhat.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211104161858.21786-1-jani.nikula@intel.com