drm/nouveau: debugfs: implement DRM GPU VA debugfs

Provide the driver indirection iterating over all DRM GPU VA spaces to
enable the common 'gpuvas' debugfs file for dumping DRM GPU VA spaces.

Reviewed-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230804182406.5222-13-dakr@redhat.com