avcodec/v4l2_m2m_enc: Check encoder pix_fmt matches pix_fmt on device
Fixes #8079
During initialization of a v4l2m2m device, the configured pix_fmt can be
different to the pix_fmt of the encoder (i.e. avctx->pix_fmt).
For example on the Odroid XU4:
./ffmpeg -f lavfi -i yuvtestsrc -codec:v h264_v4l2m2m out.h264
will configure the v4l2 encoder to pix_fmt nv21, whereas the input
frames will be yuv444p.
This commit checks that the configured v4l2 pix_fmt on device is the
same as avctx->pix_fmt. If they are different the initialization fails
and an error is returned. Tested on RPI4 and Odroid XU4.