Upcoming AMD multichannel support requires many customized operations
(channel mapping, ELD, HBR) but can otherwise share most of its code
with the generic patch.

Add a local struct hdmi_ops containing customizable HDMI-specific
callbacks and move the current code to those callbacks. Functionality is
unaltered.
Signed-off-by: Anssi Hannula <anssi.hannula@iki.fi>
Signed-off-by: Takashi Iwai <tiwai@suse.de>