In order to be consistent with the other Y'CbCr encodings add
support for full range V4L2_YCBCR_ENC_BT2020_CONST_LUM.

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