[media] vivid-tpg: don't add offset when switching to monochrome

The grayscale values are still full range sRGB, so don't add the
limited range offset.

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