drm/exynos: fix buffer pitch calculation

Change-Id: Ia0f536eec9d1f12bd8a6377f5b87807a651fd65c
Signed-off-by: Cooper Yuan <cooperyuan@gmail.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>