Replacing (raster_block >> tx_size) with (block >> (tx_size << 1)).

The new expression is much more logical than previous one. Surprisingly
both expressions give exactly the same set of dependent values
-- have_top, have_left, have_right -- in vp9_predict_intra_block.

Change-Id: I63eb1b592b8c37883b3a0dbb1f3daa271e446109