drm/i915: disable sound first on intel_disable_ddi

Our mode set sequence documentation says audio must be disabled first.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
[danvet: Resolve conflict since the first patch in this series isn't
applied yet. Also bikeshed commit message as suggested by Ben.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>