x86/amd-iommu: Cleanup completion-wait handling

This patch cleans up the implementation of completion-wait
command sending. It also switches the completion indicator
from the MMIO bit to a memory store which can be checked
without IOMMU locking.
As a side effect this patch makes the __iommu_queue_command
function obsolete and so it is removed too.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>