obs-ffmpeg: Cap NVENC Max B-frames according to GPU caps

Erroring out of NVENC init early if the Max B-frames setting was higher
than the encoder's capability causes an encoder failure on NVIDIA Pascal
(10-series) and earlier GPUs due to an unfortunate interaction between
Simple Output Mode, HEVC, and our default B-frames setting of 2. Since
we already check the Max B-frames capability of the encoder, cap at that
value instead of erroring out.

Fixes #7698.
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