video: exynos_dp: Fix get_pll_lock_status return value

Fix the return value of exynos_dp_get_pll_lock_status to
reflect what it actually returns.

Signed-off-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Olof Johansson <olofj@chromium.org>
Acked-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>