replace LOG2 with util_fast_log2

replace LOG2 with util_fast_log2

The implementation is somewhat different, although if you go back in
time far enough they're the same, but the one in u_math was changed a
long time back to be faster.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3024>