Let LMR at root be independent of MultiPV value

Let LMR at root be independent of MultiPV value

Current formula enable LMR when

i + MultiPV >= LMRPVMoves

It means that, for instance, if MultiPV == 1 then LMR
will be started to be considered at move i = LMRPVMoves - 1,
while if MultiPV == 3 then it will start before,
at move i = LMRPVMoves - 3.

With this patch the formula becomes

i >= MultiPV + LMRPVMoves - 2

So that LMR will always start after LMRPVMoves - 1 moves
from the last PV move.

No functional change when MultiPV == 1

Signed-off-by: Marco Costalba <mcostalba@gmail.com>