Move src_diff to per-plane MACROBLOCK data

First in a series of commits making certain MACROBLOCK members
addressable per-plane. This commit also refactors the block subtraction
functions vp9_subtract_b, vp9_subtract_sby_c, etc to be
loops-over-planes and variable subsampling aware.

Change-Id: I371d092b914ae0a495dfd852ea1a3d2467be6ec3