video: exynos_dp: fix build warning due to uninitialized value

This patch fixes build warning due to uninitialized value dereference.

drivers/video/exynos/exynos_dp_core.c: In function 'exynos_dp_set_link_train':
drivers/video/exynos/exynos_dp_core.c:529:18: warning: 'reg' may be used uninitialized in this function [-Wuninitialized]
drivers/video/exynos/exynos_dp_core.c:395:6: note: 'reg' was declared here

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