[media] vivid: fix test pattern movement for V4L2_FIELD_ALTERNATE

The successive TOP/BOTTOM fields did not move as they should: only
every other field actually changed position.

The cause was that the tpg needs to know if it is dealing with a
FIELD_ALTERNATE case since that requires slightly different handling.

So tell the TPG whether or not the field setting is for the ALTERNATE
case or not.

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