drm/exynos: add generic check for plane state

This patch adds generic check for plane state - display area dimensions,
so drivers can always assume that they get valid plane state to set.

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>