panthor: use drm_gpuva_unlink_defer()

panthor: use drm_gpuva_unlink_defer()

Instead of manually deferring cleanup of vm_bos, use the new GPUVM
infrastructure for doing so.

To avoid manual management of vm_bo refcounts, the panthor_vma_link()
and panthor_vma_unlink() methods are changed to get and put a vm_bo
refcount on the vm_bo. This simplifies the code a lot. I preserved the
behavior where panthor_gpuva_sm_step_map() drops the refcount right away
rather than letting panthor_vm_cleanup_op_ctx() do it later.

Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://lore.kernel.org/r/20251006-vmbo-defer-v4-2-30cbd2c05adb@google.com
Signed-off-by: Alice Ryhl <aliceryhl@google.com>