iommu/amd: Add several helper functions

Move single iommu enabling codes into a wrapper function early_enable_iommu().
This can make later kdump change easier.

And also add iommu_disable_command_buffer and iommu_disable_event_buffer
for later usage.

Signed-off-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>