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.
Signed-off-by: Andriy Gelman <andriy.gelman@gmail.com>