[media] au0828: don't hardcode height/width

While this device doesn't have a scaler (or have it disabled),
the screen dimentions are a function of the standard. Ok, right
now, only 480 lines standards are implemented, although it
supports other ones. Yet, let's calculate the size, to make
easier to add more standards latter.

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>