drm: exynos: fix for loosing display mode header during mode adjustment

This patch is to preserve the display mode header during the mode adjustment.
Display mode header is overwritten with the adjusted mode header which is
throwing the stack dump.

Signed-off-by: Rahul Sharma <rahul.sharma@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>