drm/nouveau/kms/gv100-: move window ownership setup into modesetting path

For various complicated reasons, we need to avoid sending a core update
method during display init.  Something, which we've been required to do
on GV100 and up because we've been assigning windows to heads there and
the HW is rather picky about when that's allowed.

This moves window assignment into the modesetting path at a point where
it's much safer to send our first update methods to NVDisplay.

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