Fix use of bestPredModeDCT2 in intra search
The value bestPredModeDCT2 is used in IntraSearch::updateISPStatusFromRelCU where it is interpreted to represent a "regular" intra mode index. Its value should thus not be set to an MIP mode index. Note that the MIP flag value used in IntraSearch::updateISPStatusFromRelCU may be different from the flag value when bestPredModeDCT2 is set.