drm/exynos: use DIV_ROUND_CLOSEST to find the closest div

This patch uses DIV_ROUND_CLOSEST instead of DIV_ROUND_UP
The DIV_ROUND_CLOSEST can be used to find the closest integer
value when we divide some integers.

Signed-off-by: Chanho Park <chanho61.park@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>