Reworked the code to use vp8_build_intra_predictors_mby_s,
vp8_intra_prediction_down_copy, and vp8_intra4x4_predict_d_c
functions instead.  vp8_intra4x4_predict_d_c is a decoder-only
version of vp8_intra4x4_predict.  Future commits will fix this
code duplication.

Change-Id: Ifb4507103b7c83f8b94a872345191c49240154f5