[media] vivid-tpg: correctly average the two pixels in gen_twopix()

gen_twopix() is always called twice: once for the first and once for
the second pixel. Improve the code to properly average the two if the
format requires horizontal downsampling.

This is necessary for patterns like 1x1 red/blue checkers.

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