v4l: Validate fields in the core code for subdev EDID ioctls

The subdev EDID ioctls receive a pad field that must reference an
existing pad and an EDID field that must point to a buffer. Validate
both fields in the core code instead of duplicating validation in all
drivers.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Hans Verkuil <hans.verkuil@cisco.com>