drm: Add CHANGE_HANDLE ioctl for drm gem

[David Francis]

This patch adds an ioctl to reassign the gem handle of a bo.
It was formerly the first patch of my CRIU with dmabuf patchset.

Would it be possible to merge this on its own? Internal deadlines
would prefer this to be in the next kernel release if possible.

The CRIU patches that are the current use case for this can be
found at https://github.com/checkpoint-restore/criu/pull/2613