- Implement drm_initmap, and extend it with the resource number to help

- Implement drm_initmap, and extend it with the resource number to help
    FreeBSD. Add drm_get_resource_{start|len} so linux-specific stuff
    doesn't need to be in shared code.
- Fix mach64 build by using __DECONST to work around passing a const
    pointer to useracc, which is unfortunately not marked const.
- Get rid of a lot of maplist code by not having dev->maplist be a pointer,
    and by sticking the link entries directly in drm_local_map_t rather
    than having a separate structure for the linked list.
- Factor out map uninit and removal into its own routine, rather than
    duplicating in both drm_takedown() and drm_rmmap().
- Hook up more driver functions, and correct FreeBSD-specific bits of
    radeon_cp.c, making radeon work.
- Baby steps towards using bus_space as we should.