drm/ttm: implement LRU add callbacks v2

This allows fine grained control for the driver where to add a BO into the LRU.

v2: fix typo in comment

Reviewed-by: Sinclair Yeh <syeh@vmware.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>