drm/nouveau/ttm: memcpy waits for bo already

no need for driver to wait here.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200924051845.397177-29-airlied@gmail.com