media: vimc: use correct subdev functions

Instead of calling everything a MEDIA_ENT_F_ATV_DECODER, pick the
correct functions for these blocks.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>