drm/nouveau/kms: prepare to support suspend/resume of display state with atomic

This is different from the equivilant functions in the atomic helpers in
that we fully disable the pipe instead of just setting it to inactive.

We do this (primarily) to ensure the framebuffer cleanup paths are hit,
allowing buffers to be un-pinned from memory so they can be evicted to
system memory and not lose their contents while suspended.

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