nvenc: Avoid extra bitstream buffers allocation
Submitted by Kyrylo V. Polezhaiev
Bitstream buffers allocation is very expensive operation. We shouldn't use 32 / 48, as this gives us no performance boost but makes initialisation take 2 seconds of whole CPU / memory bus, adds unacceptable latency and other operations should wait.
For shorter videos this makes nvh264enc less performant than x264enc.