drm/i915: set proper N/CTS in modeset

When modeset occurs and the TMDS frequency is set to some
speical values, the N/CTS need to be set manually if audio
is playing.

Signed-off-by: Libin Yang <libin.yang@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>