memory manager: Make device driver aware of different memory types.

Memory types are either fixed (on-card or pre-bound AGP) or not fixed
(dynamically bound) to an aperture. They also carry information about:

1) Whether they can be mapped cached.
2) Whether they are at all mappable.
3) Whether they need an ioremap to be accessible from kernel space.

In this way VRAM memory and, for example, pre-bound AGP appear
identical to the memory manager.

This also makes support for unmappable VRAM simple to implement.