media: rkvdec: Hook the (TRY_)DECODER_CMD stateless ioctls

The (TRY_)DECODER_CMD ioctls are used to support flushing when holding
capture buffers is supported. This is the case of this driver but the
ioctls were never hooked to the ioctl ops.

Add them to correctly support flushing.

Fixes: ed7bb87d ("media: rkvdec: Enable capture buffer holding for H264")
Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>