We want to be able to survive accesses to, from Vulkan's
perspective, unmapped regions of sparse resources. For that we
allocate a single page -sized bo, which we'll use to implement
sparse unmapping by mapping the address range to this bo.
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35287>