drm/nouveau: Remove unecessary dma_fence_ops

dma_fence_default_wait is the default now.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: nouveau@lists.freedesktop.org
Link: https://patchwork.freedesktop.org/patch/msgid/20180704092909.6599-4-daniel.vetter@ffwll.ch