Skip checking obmc when regular inter predictor is not so good (the
rd-cost for Y residual is greater than the total rd of the best mode
so far.)

Performance change compared to full rd search:
  +0.006% lowres, -0.056% midres
Encoding time :
  1.14X baseline (was 1.42X)

Change-Id: I11350f955a20e1a2331be458537a915e09fbedf3