drm/i915/audio: abstract audio config update

Prepare for using the same code for updating HSW_AUD_CFG register. 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/56fe0662990289c647f998c11089133ca92ebb68.1476111629.git.jani.nikula@intel.com