Renaming in MB_MODE_INFO

The macro block mode info context originally contained an
entry for each 16x16 macroblock. In VP9 each entry refers
to an 8x8 region not a macro block, so the naming is misleading.

This first stage clean up changes the names of 3 entries in the
structure to remove the mb_ prefix.

TODO clean up the nomenclature more widely in respect of
mbmi and bmi.

Change-Id: Ia7305c6d0cb805dfe8cdc98dad21338f502e49c6