i40e: enable packet split only when IOMMU present

When an IOMMU is in use, the packet split receive path shows a distinct
advantage over the single-buffer path because it minimizes DMA mapping
and unmapping. However, this is not an advantage for systems with no
IOMMU. At init time, check to see if an IOMMU is enabled and enable
packet split receives.

Change-ID: I4f70d2e9c31bbea3dc8fd0c5734959a6e6602210
Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Jim Young <james.m.young@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>