drm/i915/audio: define the audio struct separately from drm_i915_private

Add a standalone definition of struct intel_audio_private, and note that
all of it is private to intel_audio.c.

v2: Rebase

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/20211104161858.21786-3-jani.nikula@intel.com