drm/i915/ddi: write ELD where it's supposed to be done

The audio programming sequence states that the ELD must be written and
enabled after the pipe is ready. Indeed, this should clarify the
situation with

commit c7905792
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Wed Apr 16 16:56:09 2014 +0200

    drm/i915: Remove vblank wait from haswell_write_eld

and Ville's review of it [1].

Moreover, we should not touch the relevant registers before we get the
audio power domain.

[1] http://mid.gmane.org/20140416155309.GK18465@intel.com



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>