drm/ttm: make common function for wait/free node path.

The pipeline and accel cleansups has similiar paths here.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200917043040.146575-8-airlied@gmail.com