Rework the ref_frame_skip_mask feature in RDO

Rework the ref_frame_skip_mask feature in RDO

Previously we often skip all compound inter prediction modes,
causing large coding loss. This patch modifies how we set the
ref_frame_skip_mask so that compound modes are considered in RDO.

This affects speed>=1.

Coding gains(overall psnr):
          lowres       midres     hdres     average
speed 1    0.54%       0.43%      0.64%      0.53%
speed 2    0.59%       0.48%      0.60%      0.56%

Tested encoding speed on 10 HD sequences, average speed loss is
5% for speed 1; 2% for speed 2.

Change-Id: Ib8758af7ee7c9812022bd21c5fe61631e2bb8e5c