stop overriding dma_ops in vmd v2

[Christoph Hellwig]

Hi all,

this is a new version of the vmd dma_map_ops removal, which does not
require vmd to be built in.  Instead we slightly expand the vmd-specific
field in the x86 pci_sysdata to cover that information.

Note that I do not have a vmd-enable system, so some testing by the
maintainers would be welcome.