preset default set 'p2' when use nvenc_hevc encoder will get error

[Yshelgi]

When i use hevc as videowrite encoder and not set preset,i get error as pic

from this error, i track the code and find the default preset = p2 in ffmpeg_writer.py

I use ffmpeg -h encoder=nvenc_hevc to check preset,but 'p2' is not in options

In order to solve this problem, either force the preset parameter to be set, or modify the default value setting here in the source code.