Frequent IOMMU remappings take about 50% of CPU usage because there is
quite a lot to remap. Defer dmabuf's unmapping by 5 seconds in order to
mitigate the mapping overhead which goes away completely and driver works
as fast as in a case of a disabled IOMMU. The case of a disabled IOMMU
should also benefit a tad from the caching since CPU cache maintenance
that happens on dmabuf's attaching takes some resources.