iommu/omap: Reset the domain field upon detaching

The .domain field in omap_iommu struct is set properly when the
OMAP IOMMU device is attached to, but is never reset properly
on detach. Reset this properly so that the OMAP IOMMU debugfs
logic can depend on this field before allowing the debugfs
operations.

Signed-off-by: Suman Anna <s-anna@ti.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>