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 <[email protected]>
Cc: Kai Vehmanen <[email protected]>
Cc: Takashi Iwai <[email protected]>
Reviewed-by: Jani Nikula <[email protected]>
Reviewed-by: Kai Vehmanen <[email protected]>
Signed-off-by: Ville Syrjälä <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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