loop_filter_rows_mt: unify worker count calculation

fixes a deadlock with an odd number of threads that go from < number of
tiles to >. the previous calculations were out of sync so going from
e.g., 8 tiles to 2 with 3 threads would result in scheduling only 2
workers, but thread_loop_filter_rows() would expect 3.

BUG=webm:1618

Change-Id: I78c967a8c3c927d929e13c949808a5ef443ebacb