drm/nouveau/kms/nv50-: clear SW state of disabled windows harder

The most innocuous result of not having done this is that we end up
sending unnecessary methods when we next enable the window.

However, interactions with the code handling skipping disables when
an update immediately follows, and window ownership assignment, can
lead to upsetting the display hardware on Volta and newer.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>