media: v4l2-ctrl: Move compound control validation

Rework std_validate moving the compound controls to
its own validation function.

While here, fix the pointer math to account the index parameter.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>