drm/exynos: exynos7-decon: remove excessive check

Display area is already checked by exynos plane core, so there is no
need for such check in driver code.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Inki Dae <inki.dae@samsung.com>