ttm: Fix caching mode selection.

A bug caused a new caching state to be selected on each buffer object
validation regardless of the current caching state.
Moreover, a caching state could be selected that wasn't supported by
the memory type.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>