Clamp_mv_ref happens in multiple places in ref_mv search, which can be
convoluted as reported in issue 1124. This change is to consolidate
the clamping into one place.

Borg test result on lowres set:
avg_psnr:    ovr_psnr:   ssim:
  0.000       0.000      0.001

BUG=aomedia:1377
BUG=aomedia:1124
BUG=aomedia:857

Change-Id: I1649d5b5f37683c9c30e493c6eed13a808ab543a