cuvid: support AVCodecContext.hw_device_ctx API

This is a newer API that is intended for decoders like the cuvid
wrapper. Until now, the wrapper required to set an awkward
"incomplete" hw_frames_ctx to set the device. Now the device
can be set directly, and the user can get AV_PIX_FMT_CUDA output
for a specific device simply by setting hw_device_ctx.

This still does a dummy ff_get_format() call at init time, and should
be fully backward compatible.