drm/ttm: remove default caching

As far as I can tell this was never used either and we just
always fallback to the order cached > wc > uncached anyway.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/390142/