drm/i915/audio: split dp and hdmi audio config update

The code for dp and hdmi are already different, and they're about to
diverge even more. Split them for clarity in future work. No functional
changes.

Cc: Libin Yang <libin.yang@linux.intel.com>
Reviewed-by: Libin Yang <libin.yang@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/41b8e24fed92effafaef69675ddabfa2008b4d31.1476111629.git.jani.nikula@intel.com