MIPS: dspr2: added optimization for Disto4x4

enc/dec common macros moved to mips_macro.h

Change-Id: I38d491e772554ac663dd5eb4d15485c0343f23b1
FlattenSimilarBlocks should only be tried when blending is possible.

This is because, FlattenSimilarBlocks() replaces some opaque pixels by
transparent ones. This results in an equivalent output only if blending
is turned on for the current frame.

Change-Id: I05612c952fdbd4b3a6e0ac9f3a7d49822f0cfb9b
Merge "FlattenSimilarBlocks should only be tried when blending is possible."