zbva is always false, so fbo is never read.

A 'zero-based-virtual-address' is simply IOVA == 0, and the driver already
supports this.

Link: https://lore.kernel.org/r/16-v2-270386b7e60b+28f4-umem_1_jgg@nvidia.com
Acked-by: Michal Kalderon <michal.kalderon@marvell.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>