iommu: Simplify init function

iommu_group_ida and iommu_group_mutex can be initialized statically.
There's no need to do this dynamically in the init function.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>