We only need to try to lock a BO if it's external as non-external BOs
share the dma-resv with the already locked VM. Trying to lock
non-external BOs caused an issue (list corruption) in an uncoming patch
which adds bulk LRU move. Since this code isn't needed, remove it.

v2: New commit message, s/mattthew/matthew/
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>