Avoid unnecessary chroma MC in SAD pre-selection for GPM-TM

Avoid invoking unnecessary chroma motion compensation for all possible MVs in SAD pre-selection stage for GPM-TM. The chroma samples will anyway be generated for those selected MVs at a later stage but before entering the SSE-based RDO.

No impact on BD-rate results.

Had run some class D sequences under RA comparing to the master just to double-check.