agp/intel: add ValleyView AGP driver

... and bind it right to the PCI id.

Note that there are still a few things to fix here:
- we need to move the tlb flush to a better place in drm/i915.
- we need to check snoop support on vlv and implement it.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
[danvet: squash follow-on patch and add todo items to commit msg.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>