iommu/omap: Fix bus error on debugfs access of unattached IOMMU

Any debugfs access on an OMAP IOMMU that is not enabled (done during
attach) results in a bus error due to access of registers without
the clock or the reset enabled for the respective IOMMU. So, add a
check to make sure the IOMMU is enabled/attached by a client device.
This gracefully prints a "Operation not permitted" trace when the
corresponding IOMMU is not enabled.

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>