Change the MGA initialization and cleanup a bit. The dev_private structure

Change the MGA initialization and cleanup a bit. The dev_private structure
    is now allocated (and partially filled in) by the new
    mga_driver_preinit function.
This allows the driver to detect the type of card (i.e., G200 class vs.
    G400 class) on its own. The chipset value passed to mga_dma_init is now
    ignored. This same technique is used by the radeon DRM.
As a result of this, mga_driver_pretakedown was converted to
    mga_driver_postcleanup. This routine gets called in some other places
    than might be expected, and it sets the dev_private pointer to NULL.
    That little gem took over an hour to track down. :(