[media] vivid: Y offset should depend on quant. range

When converting to or from Y'CbCr and R'G'B' the Y offset depends
on the quantization range: it's 0 for full and 16 for limited range.
But in the code it was hardcoded to 16. This messed up the brightness
of the generated pattern.

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