drm/ttm: add caching state to ttm_bus_placement

And implement setting it up correctly in the drivers.

This allows getting rid of the placement flags for this.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Link: https://patchwork.freedesktop.org/patch/394254/