AMD/IOMMU: redo awaiting of command completion

AMD/IOMMU: redo awaiting of command completion

The present abuse of the completion interrupt does not only stand in the
way of, down the road, using it for its actual purpose, but also
requires holding the IOMMU lock while waiting for command completion,
limiting parallelism and keeping interrupts off for non-negligible
periods of time. Have the IOMMU do an ordinary memory write instead of
signaling an otherwise disabled interrupt (by just updating a status
register bit).

Since IOMMU_COMP_WAIT_I_FLAG_SHIFT is now unused and
IOMMU_COMP_WAIT_[FS]_FLAG_SHIFT already were, drop all three of them
while at it.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Paul Durrant <paul@xen.org>