drm/i915: introduce intel_audio_codec_{enable, disable}

Introduce functions to enable/disable the audio codec, incorporating the
ELD setup within enable. The disable is initially limited to HSW,
covering exactly what was done previously.

The only functional difference is that ELD valid is no longer set if
there is no connector with ELD, which should be the right thing to do
anyway. Otherwise the sequence remains the same, with warts and all, in
preparation for applying more sanity.

v2: add kernel doc.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>