media: v4l2-mem2mem: add try_en/decoder_cmd ioctl helpers

Most if not all codecs will need to implement these ioctls and
it is expected to be the same for all codecs. So add this to
the core v4l2-mem2mem framework so that this code can easily be
reused.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>