intel-iommu: Kill DMAR_BROKEN_GFX_WA option.

Just make it depend on BROKEN for now, in case people scream really loud
about it (and because we might want to keep some of this logic for an
upcoming BIOS workaround, so I don't just want to rip it out entirely
just yet). But for graphics devices, it really ought to be unnecessary.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>