drm/ttm: add BO priorities for the LRUs

This way the driver can specify a priority for a BO which has the effect that
a BO is only evicted when all other BOs with a lower priority are evicted
first.

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