lavc/cook: get rid of wasteful pow in init_pow2table

The table is highly structured, so pow (or exp2 for that matter) can entirely
be avoided, yielding a ~ 40x speedup with no loss of accuracy.

sample benchmark (Haswell, GNU/Linux):
new:
4449 decicycles in init_pow2table(loop 1000),     254 runs,      2 skips
4411 decicycles in init_pow2table(loop 1000),     510 runs,      2 skips
4391 decicycles in init_pow2table(loop 1000),    1022 runs,      2 skips

old:
183673 decicycles in init_pow2table(loop 1000),     256 runs,      0 skips
182142 decicycles in init_pow2table(loop 1000),     512 runs,      0 skips
182104 decicycles in init_pow2table(loop 1000),    1024 runs,      0 skips

Reviewed-by: Clément Bœsch <u@pkh.me>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>