driver core: Introduce device_iommu_mapped() function
Some places in the kernel check the iommu_group pointer in
'struct device' in order to find out whether a device is
mapped by an IOMMU.
This is not good way to make this check, as the pointer will
be moved to 'struct dev_iommu_data'. This way to make the
check is also not very readable.
Introduce an explicit function to perform this check.