Rework TTMs busy handling

[Christian König]

Hi guys,

so pushed the first few patches from this series. I hope that I
correctly managed to resolve the silent Xe merge conflict in drm-tip,
but would be nice if somebody could double check.

Then for the two remaining patches I've implemented most of what
Thomas suggest, e.g. the existing functionality sticks around for
eviction and hobs, but ttm_bo_validate will now try to always move
things into the non-fallback placements on validation first.

What I haven't done yet is to split up the preferred placement since
I couldn't immediately see an use case for this, but it's really
something we might do in the future as well.

Please review and comment,
Christian.