drm/nouveau: uvmm: fix unset region pointer on remap

Transfer the region pointer of a uvma to the new uvma(s) on re-map to
prevent potential shader faults when the re-mapped uvma(s) are unmapped.

Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230820222920.2344-1-dakr@redhat.com