vgaarb: provide a vga_client_unregister wrapper

Add a trivial wrapper for the unregister case that sets all fields to
NULL.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210716061634.2446357-6-hch@lst.de


Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>