Rework drm prime guts.

- Our faux struct sg_table is now just an array of _pages_, not of
  bus dma segments.

- We handle bus dma segments only in the presence of an adult bus dma
  tag.

- All bus dma hacks are once again confined to bus_dma_hacks.h, and
  more neatly organized:
  . bus_dmamem_export_pages translates bus dma segs to vm_page array
  . bus_dmamem_import_pages translates vm_page array to bus dma segs

(riastradh)