b_width_log2 and b_height_log2 lookups

    Replace case statement with lookup.
    Small speed gain at low speed settings but at speed 2+ where the
    number of motion searches etc. falls the impact rises to ~3-4%.

    Change-Id: Idff639b7b302ee65e042b7bf836943ac0a06fad8

Change-Id: I5940719a4a161f8c26ac9a6753f1678494cec644