iommu/omap: Change the attach detection logic

The OMAP IOMMU driver allows only a single device (eg: a rproc
device) to be attached per domain. The current attach detection
logic relies on a check for an attached iommu for the respective
client device. Change this logic to use the client device pointer
instead in preparation for supporting multiple iommu devices to be
bound to a single iommu domain, and thereby to a client device.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>