drm/i915/audio: Protect singleton register with a lock

On the "ilk" platforms AUD_CNTL_ST2 is a singleton. Protect
it with the audio mutex in case we ever want to do parallel
RMW access to it.

Currently that should not happen since we only do audio
enable/disable from full modesets, and those are fully
serialized. But we probably want to think about toggling
audio on/off from fastsets too.

The hsw codepaths already have the same locking.
g4x should not need it since it can only do audio to a
single port at a time, which means it's actually broken
in more ways than this atm.

Cc: Chaitanya Kumar Borah <chaitanya.kumar.borah@intel.com>
Cc: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Cc: Takashi Iwai <tiwai@suse.de>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221026170150.2654-8-ville.syrjala@linux.intel.com