drm/nouveau: use DRM_FB_HELPER_DEFAULT_OPS for fb_ops

Cc: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Stefan Christ <contact@stefanchrist.eu>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1479078208-25221-9-git-send-email-contact@stefanchrist.eu