drm/exynos: add ratio calculation

Calculation ratio from exynos_drm plane codes, then each hw drivers can
use it without extra operation. Also this fixes width and height of
source used for actual crtc shown via screen.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>