drm/ttm: revert "implement LRU add callbacks v2"

The additional housekeeping had too much CPU overhead,
let's use the BO priorities instead.

agd: also revert hibmc changes

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