[media] vivid-tpg: add helper functions for single buffer planar formats

Add helpers functions to determine the line widths and image sizes
for planar formats that are stores in a single buffer.

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