drm/exynos: Replaced kzalloc & memcpy with kmemdup

Replaced calls to kzalloc followed by memcpy with call to kmemdup.
Patch found using coccinelle.

Signed-off-by: Alexandru Gheorghiu <gheorghiuandru@gmail.com>
Acked-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>