drm/nouveau/kms: subclass atomic connector state

This commit implements the atomic property hooks for a connector, and
wraps the legacy interface handling on top of those.

For the moment, a full modeset will be done after any property change
in order to ease subsequent changes.  The optimised behaviour will be
restored for Tesla and later (earlier boards always do full modesets)
once atomic commits are implemented.

Some functions are put under the "nouveau_conn" namespace now, rather
than "nouveau_connector", to distinguish functions that will work for
(upcoming) MST connectors too.

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