drm/i915: Hoist the encoder->audio_{enable,disable}() calls higher up

Push the encoder->audio_{enable,disable}() calls out from the
encoder->{enable,disable}() hooks. Moving towards audio fastset.

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/20231121054324.9988-10-ville.syrjala@linux.intel.com