[media] videodev2.h: add support for transfer functions

In the past the transfer function was implied by the colorspace. However,
it is an independent entity in its own right. Add support for explicitly
choosing the transfer function.

This change will allow us to represent linear RGB (as is used by openGL), and
it will make it easier to work with decoded video material since most codecs
store the transfer function as a separate property as well.

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