[media] vivid-tpg: improve Y16 color setup

Currently the colors for the Y16 and Y16_BE pixelformats are in the range
0x0000-0xff00. So pure white (0xffff) is never created.

Improve this by making white really white. For other colors the lsb remains 0
so vivid can be used to detect endian problems.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Ricardo Ribalda <ricardo.ribalda@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>