media: v4l2-ctrls: alloc arrays in ctrl_ref

Also allocate space for arrays in struct ctrl_ref.

This is in preparation for allowing to change the array size from
a driver.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>