iommu/amd: Introduce protection_domain_init() function

This function contains the common parts between the
initialization of dma_ops_domains and usual protection
domains. This also fixes a long-standing bug which was
uncovered by recent changes, in which the api_lock was not
initialized for dma_ops_domains.

Reported-by: George Wang <xuw2015@gmail.com>
Tested-by: George Wang <xuw2015@gmail.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>