[media] videodev2.h: defines to calculate blanking and frame sizes

It is very common to have to calculate the total width and height of the
blanking and the full frame, so add a few defines that deal with that.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>