agp/uninorth: Fix lockups with radeon KMS and >1x.

This was based on a description by Ben Herrenschmidt:

> I've removed that SBA reset from the normal TLB invalidation path and
> left it only once after turning AGP on.

About six months ago, he said:

> I did it a bit differently, but yeah, you get the idea. I'm doing a
> patch series so don't bother pushing things too hard yet.

But I haven't seen anything from him about this since then, and people are
regularly hitting these lockups, so here we are...

Signed-off-by: Michel Dänzer <daenzer@vmware.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Dave Airlie <airlied@gmail.com>