x86/tboot: Don't disable swiotlb when iommu is forced on

After commit 327d5b2f ("iommu/vt-d: Allow 32bit devices to uses DMA
domain"), swiotlb could also be used for direct memory access if IOMMU
is enabled but a device is configured to pass through the DMA translation.
Keep swiotlb when IOMMU is forced on, otherwise, some devices won't work
if "iommu=pt" kernel parameter is used.

Fixes: 327d5b2f

 ("iommu/vt-d: Allow 32bit devices to uses DMA domain")
Reported-and-tested-by: Adrian Huang <ahuang12@lenovo.com>
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Link: https://lore.kernel.org/r/20201125014124.4070776-1-baolu.lu@linux.intel.com
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=210237


Signed-off-by: Will Deacon <will@kernel.org>