drm/i915: Convert audio enable/disable into encoder vfuncs

Add encoder vfuncs for audio enable/disable. This will allow
audio to be enabled/disabled during fastsets. An encoder hook
is necessary as on pre-hsw platforms different encoder types
implement audio in different ways.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231121054324.9988-9-ville.syrjala@linux.intel.com