Revert "drm/ttm: add a way to bo_wait for either the last read or last write"

This reverts commit dfadbbdb57b3f2bb33e14f129a43047c6f0caefa.

Further upstream discussion between Marek and Thomas decided this wasn't
fully baked and needed further work, so revert it before it hits mainline.

Signed-off-by: Dave Airlie <airlied@redhat.com>