Skip mode check when mv has been tested

This commit allows the non-RD mode decision to skip mode RD modelling
check, if the motion vector associated with the current mode is
same as that of NEARESTMV mode. This makes speed -7 about 2% faster.
Previous change that converts cost metric from SAD to model based RD
value makes the codec 6% slower at speed -7.

Change-Id: I30cfec5452f606a671b8432a2f7f0c94fbb49fc8