drm/nouveau/kms/nv50: skip core channel cursor update on position-only changes

The DRM core used to only call prepare_fb/cleanup_fb() when a plane's
framebuffer changed, which achieved the desired effect.

It's apparently now up to the driver to decide on its own.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Cc: stable@vger.kernel.org [4.11+]