drm/i915: fix kernel-doc warnings in intel_audio.c

drm/i915: fix kernel-doc warnings in intel_audio.c

Fix the following 'make htmldocs' warnings:

  .//drivers/gpu/drm/i915/intel_audio.c:439: warning: No description found for parameter 'intel_encoder'
  .//drivers/gpu/drm/i915/intel_audio.c:439: warning: Excess function parameter 'encoder' description in 'intel_audio_codec_disable'
  .//drivers/gpu/drm/i915/intel_audio.c:439: warning: No description found for parameter 'intel_encoder'
  .//drivers/gpu/drm/i915/intel_audio.c:439: warning: Excess function parameter 'encoder' description in 'intel_audio_codec_disable'

Signed-off-by: Geliang Tang <geliangtang@163.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>