avcodec/nvenc: fix lossless tuning logic

avcodec/nvenc: fix lossless tuning logic

Relying on the order of the enum is bad.
It clashes with the new presets having to sit at the end of the list, so
that they can be properly filtered out by the options parser on builds
with older SDKs.

So this refactors nvenc.c to instead rely on the internal NVENC_LOSSLESS
flag. For this, the preset mapping has to happen much earlier, so it's
moved from nvenc_setup_encoder to nvenc_setup_device and thus runs
before the device capability check.