vp9_prob_diff_update_savings_search_model: quiet conv warnings

under Visual Studio:
Warning C4244 '=': conversion from 'int64_t' to 'vpx_prob', possible loss of
data

after:
ea042a676 vp9 encoder: fix integer overflows

'newp' has already been range checked earlier in the loop so the cast won't
have any unexpected results

Change-Id: Ic10877db2c0633d53fffdf8852d5095403c23a02
1 file changed