video: exynos_dp: change return type of exynos_dp_init_video to void

This patch changes return type of exynos_dp_init_video to void,
because the return value is unnecessary.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>