drm/ttm/ttm_bo: Fix incorrectly documented function 'ttm_bo_cleanup_refs'

drm/ttm/ttm_bo: Fix incorrectly documented function 'ttm_bo_cleanup_refs'

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/ttm/ttm_bo.c:293: warning: expecting prototype for function ttm_bo_cleanup_refs(). Prototype was for ttm_bo_cleanup_refs() instead

Cc: Christian Koenig <christian.koenig@amd.com>
Cc: Huang Rui <ray.huang@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-media@vger.kernel.org
Cc: linaro-mm-sig@lists.linaro.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210416143725.2769053-24-lee.jones@linaro.org
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>