drm/i915/audio: s/dev_priv/i915/

Rename the 'dev_priv' variables to 'i915' in the audio code
to match modern style conventions.

v2: Drop some needless braces in intel_audio_hooks_init()

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/20221026170150.2654-2-ville.syrjala@linux.intel.com