Some V4L2 drivers fail to allocate buffers when sizeimage is not set
to a max value. This is indeed the case for s5p-mfc [1]

Most drivers should be able to calculate this value from the frame
dimensions and format - or at least have their own default.

However since this work around should not impact those drivers doing
the "right thing" this commit just provides such a default.

The calculations were extracted from the v4l2 driver used to develop
the ffmpeg v4l2_m2m support [2]. See venc.c and vdec.c

[1] linux.git/drivers/media/platform/s5p-mfc
[2] linux.git/drivers/media/platform/qcom/venus/