iommu/vt-d: Refactor Kconfig a bit

Put all sub-options inside a "if INTEL_IOMMU" so that they don't need to
always depend on INTEL_IOMMU. Use IS_ENABLED() instead of #ifdef as well.

Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Link: https://lore.kernel.org/r/20210720013856.4143880-1-baolu.lu@linux.intel.com
Link: https://lore.kernel.org/r/20210818134852.1847070-4-baolu.lu@linux.intel.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>