These multiplications convert from mi units to subpels (1/8th pixel)
and weren't correct if MI_SIZE was 4 rather than 8. This patch
rephrases the multiplications as x * 8 * MI_SIZE or x/2 * 8 *
MI_SIZE (for obmc calculations where you need the middle of a block).

Change-Id: I7997b1221ac417a223a94f1f26a86b6acd27c5da