drm/nouveau: remove open-coded drm_invalid_op()

Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: nouveau@lists.freedesktop.org
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190522150219.13913-2-emil.l.velikov@gmail.com