media: v4l2: Make colorspace validity checks more future-proof

The helper functions that test validity of colorspace-related fields
use the last value of the corresponding enums. This isn't very
future-proof, as there's a high chance someone adding a new value may
forget to update the helpers. Add new "LAST" entries to the enumerations
to improve this, and keep them private to the kernel.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>