[media] videodev2.h: remove 'experimental' annotations

Most of what is marked as 'experimental' has been around for years. Time
to drop that annotation.

The only remaining 'experimental' bits of the API are the debug ioctls
and structs: these should remain experimental since the only application
that should use this is v4l2-dbg.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>