drm/i915: Avoid MST pipe handling for LPE audio

The pipe gets cleared to -1 for non-MST before the ELD audio
notification due to the MST audio support.  This makes sense for
HD-audio that received the MST handling, but it's useless for LPE
audio.  Handle the MST pipe hack conditionally only for HD-audio.

Reported-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>