drm/amdgpu: Support dmabuf IPC with amdgpu CRIU (v3)

[David Francis]

This patch series adds support for CRIU checkpointing of processes that
share memory with the amdgpu dmabuf interface.

In this v3, the drm interfaces have been changed from creating buffer objects
with specified gem handles to changign the gem handle of an existing buffer
object.

In the accompanying CRIU patch set, the sockets used by CRIU to exchange dmabuf
fds between restoring processes have been refactored to be created within the
amdgpu plugin itself.