drm/audio: declaration of struct device

Header has used the references to struct device without it definition
or declaration. Hence resulting in compilation warning such as

  "'struct device' declared inside parameter list..."

This changes adds a declaration to struct device in the header to avoid
any such warnings.

Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
cc: Takashi Iwai <tiwai@suse.de>
cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1550293499-5560-1-git-send-email-ramalingam.c@intel.com