drm/ttm: use caching instead of placement for ttm_io_prot

Instead of the placement flags use the caching of the bus
mapping or tt object for the page protection flags.

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/394255/