iommu/iommu-debug: Maintain list of domains during alloc

Current list of domains in iommu-debug was only
maintained during attach/detach calls. But for
masters like graphics this won't account for all
the domains, as it allocates multiple different
domains but attaches only one domain at a time.

Add support for maintaining list of unattached
domains too by adding them to debug_attachments
list during domain alloc but keeping dev as NULL.
We would add entry in debugfs attachment directory
only on actual attach call.

Change-Id: Ifde043e5c39f356b4187a30cbdf020ee943618f1
Signed-off-by: Susheel Khiani <skhiani@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>