drm/exynos: mixer: unify a check for video-processor window

Always use macro instead of hard-coded '2' value in conditions related
to video processor window. Additional checks are not needed, because
video layer is registered only when video processor is available.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>