iommu/ipmmu-vmsa: Fix IOMMU lookup when multiple IOMMUs are registered

When adding a new device the driver loops over all registered IOMMUs and
calls the ipmmu_find_utlbs() function to parse the DT iommus attribute.
The function returns an error when the IOMMU referenced in DT doesn't
match the current IOMMU. The caller incorrectly breaks from the loop
immediately when the error is reported, resulting in only the first
IOMMU being considered.

Fix this, and while at it move code that isn't specific to an IOMMU
instance out of the loop.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>