drm/exynos: Use PTR_ERR_OR_ZERO in exynos_dp_core.c

PTR_ERR_OR_ZERO simplifies the code.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>