[media] s5p-fimc: Reuse calculated sizes

This formula did not take into account the required tiled alignement for
NV12MT format. As this was already computed an stored in payload array
initially, reuse that value.

Signed-off-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>