[media] v4l2: add VIDIOC_(TRY_)DECODER_CMD

As discussed during the 2011 V4L-DVB workshop, the API in dvb/video.h should
be replaced by a proper V4L2 API. This patch turns the VIDEO_(TRY_)DECODER_CMD
ioctls into proper V4L2 ioctls.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>