drm/i915/audio: rename N value getter to emphasize it's for hdmi

We'll be getting a function and a table for dp parameters soon enough,
so rename the function and table for hdmi. 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/3d1c61cab70b6a2966db9b6115b76edbd747a835.1476111629.git.jani.nikula@intel.com