intel-iommu: Restore DMAR_BROKEN_GFX_WA option for broken graphics drivers

We need to give people a little more time to fix the broken drivers.
Re-introduce this, but tied in properly with the 'iommu=pt' support this
time. Change the config option name and make it default to 'no' too.

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