drm/nouveau: bo: initialize GEM GPU VA interface

Initialize the GEM's DRM GPU VA manager interface in preparation for the
(u)vmm implementation, provided by subsequent commits, to make use of it.

Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230804182406.5222-6-dakr@redhat.com