drm/i915: rewrite hsw/bdw audio codec enable/disable sequences

There's some serious confusion regarding ELD valid bit that gets set and
cleared back and forth etc. Rewrite it all based on the documented audio
codec enable/disable sequences.

v3: replace vblank wait with a comment

v4: expand the comment on what should be done with the vblank wait

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>