iommu/vt-d: Remove BUG_ON when domain->pgd is NULL

When performing domain_context_mapping or getting dma_pte of a pfn, the
availability of the domain page table directory is ensured. Therefore,
the domain->pgd checkings are unnecessary.

Signed-off-by: Tina Zhang <tina.zhang@intel.com>
Link: https://lore.kernel.org/r/20230406065944.2773296-5-tina.zhang@intel.com


Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>