[media] vivid-tpg: split off the pattern drawing code

The last part of the vivid-tpg refactoring: split off the pattern
drawing code into a function of its own. This greatly improves the
readability and maintainability of this code.
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>