iommu/vt-d: Allow 32bit devices to uses DMA domain

Currently, if a 32bit device initially uses an identity domain, Intel
IOMMU driver will convert it forcibly to a DMA one if its address
capability is not enough for the whole system memory. The motivation was
to overcome the overhead caused by possible bounced buffer.

Unfortunately, this improvement has led to many problems. For example,
some 32bit devices are required to use an identity domain, forcing them
to use DMA domain will cause the device not to work anymore. On the
other hand, the VMD sub-devices share a domain but each sub-device might
have different address capability. Forcing a VMD sub-device to use DMA
domain blindly will impact the operation of other sub-devices without
any notification. Further more, PCI aliased devices (PCI bridge and all
devices beneath it, VMD devices and various devices quirked with
pci_add_dma_alias()) must use the same domain. Forcing one device to
switch to DMA domain during runtime will cause in-fligh DMAs for other
devices to abort or target to other memory which might cause undefind
system behavior.

With the last private domain usage in iommu_need_mapping() removed, all
private domain helpers are also cleaned in this patch. Otherwise, the
compiler will complain that some functions are defined but not used.

Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Tested-by: Daniel Drake <drake@endlessm.com>
Reviewed-by: Jon Derrick <jonathan.derrick@intel.com>
Reviewed-by: Jerry Snitselaar <jsnitsel@redhat.com>
Cc: Daniel Drake <drake@endlessm.com>
Cc: Derrick Jonathan <jonathan.derrick@intel.com>
Cc: Jerry Snitselaar <jsnitsel@redhat.com>
Link: https://lore.kernel.org/r/20200506015947.28662-2-baolu.lu@linux.intel.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>