media: v4l2-mem2mem: add stateless_(try_)decoder_cmd ioctl helpers

These helpers are used by stateless codecs when they support multiple
slices per frame and hold capture buffer flag is set. It's expected that
all such codecs will use this code.

Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Co-developed-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>