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