drm/i915/audio: name the audio sub-struct in drm_i915_private

Add name to the audio sub-struct in drm_i915_private, and remove the
tautologies and other inconsistencies in the member names.

v2: Call the mutex member mutex, not lock. (Ville)

Cc: Dave Airlie <airlied@redhat.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211104161858.21786-2-jani.nikula@intel.com