drivers: gpu: drm: i915L intel_lpe_audio: Fix kerneldoc comments

Add description for existing parameter 'pipe' to fix the build
warning: ./drivers/gpu/drm/i915/intel_lpe_audio.c:342: warning: No
description found for parameter 'pipe'.

Signed-off-by: Tamara Diaconita <diaconita.tamara@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170330115510.14054-1-diaconita.tamara@gmail.com