Fix the returned distortion value in rd_pick_intra

Return the distortion value in vp9_rd_pick_intra_mode_sb as sum of
dist_y and dist_uv. Remove the right shift operation on dist_uv,
and make it consistent with that of vp9_rd_pick_inter_mode_sb.

Change-Id: I9d564e242d9add38e32595d33b0e0dddb1d55e5b