[media] vimc: common: Add vimc_link_validate

All links will be checked in the same way. Adding a helper function for
that

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>