media: hantro: Implement support for encoder commands

The V4L2 stateful encoder uAPI specification requires that drivers
support the ENCODER_CMD ioctl to allow draining of buffers. This
however was not implemented, and causes issues for some userspace
applications.

Implement support for the ENCODER_CMD ioctl using v4l2-mem2mem helpers.
This is entirely based on existing code found in the vicodec test
driver.

Fixes: 775fec69 ("media: add Rockchip VPU JPEG encoder driver")
Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>
Reviewed-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>