drm/nouveau: allow accelerated buffer moves even when gr isn't present

There's no need to avoid using copy engines if gr init fails for some
reason (usually missing FW, or incomplete bring-up).

It's not terribly useful for an end-user, but it'll slightly speed up
suspend/resume when saving fb contents, and allow for host/ce code to
be validated.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>