[media] vivid-tpg: move the 'extras' drawing to a separate function

This moves the drawing code for the extras (border, square, etc) to
a function of its own instead of having this in the main for loop.

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