Define POSITION to differentiate from MV

MV struct was ussed to indicate the postition of a MI_BLOCK with row
and col components. The expression was confusing, this commit added a
new stucture "POSITION" with row and col component to better describe
the position of a mi_block.

Change-Id: I59fdd4b45010fe7d85a8db22a55503265c4f5b2b