Warped_motion doesn't go through inter_predictor() function to
generate prediction where convolve_round is activated.
Therefore, we could avoid doing post rounding when the
warped_motion mode is on.

We use do_post_rounding to indicate whether post rounding is
needed
Change-Id: Ib17a0c0d4e6e5232e9682afa8cfdb9576f5cf4c0