drm/i915/audio: Unify get_saved_enc()

drm/i915/audio: Unify get_saved_enc()

Make the two branches of get_saved_enc() look alike. Currently
they look different even though they do exactly the same thing
apart from == vs. != for the MST comparison.

Cc: Chaitanya Kumar Borah <chaitanya.kumar.borah@intel.com>
Cc: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Cc: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221108151839.31567-4-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>