Minorly modify model_rd_for_sb_y function

Added a skip_dc check. If skip_dc = 1, we could eliminate calling
of vp9_model_rd_from_var_lapndz(). This gave slight PSNR & SSIM
gain(<0.1%), and no speed change.

Change-Id: If5ca733366148c86b98e196a00cc890f50e9a3e5
1 file changed