drm/nouveau/kms/nv50: fix source-rect-only plane updates

This "optimisation" (which was originally meant to skip updating cursor
settings in the core channel on position-only updates) turned out to be
pointless in the final design of the code before it was merged.

Remove it completely, as it breaks other cases.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Cc: stable@vger.kernel.org [4.10+]