lavc/dsd_tablegen: speed up table generation

lavc/dsd_tablegen: speed up table generation

Tables are bit identical.
Sample benchmark (Haswell, GNU/Linux+gcc):
old:
 814485 decicycles in dsd_ctables_tableinit,     512 runs,      0 skips

new:
 356808 decicycles in dsd_ctable_tableinit,     512 runs,      0 skips

Binary size should essentially be identical, and is in fact identical on
the configuration I tested on.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>