drm/i915: split audio functions from display vtable

These are only used internally in the audio code

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/d12257cc7685a9b52618f7da444ba1fc8848b4db.1632869550.git.jani.nikula@intel.com