[media] vimc: common: Add vimc_ent_sd_* helper

As all the subdevices in the topology will be initialized in the same
way, to avoid code repetition the vimc_ent_sd_{register, unregister}
helper functions were created

Signed-off-by: Helen Koike <helen.koike@collabora.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>