Convert BSD code to mostly use bus_dma, the dma abstraction for dealing

Convert BSD code to mostly use bus_dma, the dma abstraction for dealing
    with IOMMUs and such. There is one usage of the forbidden vtophys()
    left in drm_scatter.c which will be fixed up soon. This required a KPI
    change for drm_pci_alloc/free() to return/use a drm_dma_handle_t that
    keeps track of os-specific bits, rather than just passing around the
    vaddr/busaddr/size.
Submitted by: Tonnerre Lombard (partially) Tested on: FreeBSD: Rage128
    AGP/PCI Linux: Savage4 AGP/PCI