media: vimc: add vimc_ent_type struct for the callbacks of entities

Since each vimc entity type is defined by the
callbacks implementation, it is a good idea to
add a struct to hold these callbacks.

Each vimc entity then declare its type in the file for
the entity.

Signed-off-by: Dafna Hirschfeld <dafna.hirschfeld@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>