drm/exynos: Add missing of.h header include

Add of.h explicitly for of_* APIs.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
diff --git a/drivers/gpu/drm/exynos/exynos_ddc.c b/drivers/gpu/drm/exynos/exynos_ddc.c
index 30ef41b..d1e539b 100644
--- a/drivers/gpu/drm/exynos/exynos_ddc.c
+++ b/drivers/gpu/drm/exynos/exynos_ddc.c
@@ -15,7 +15,7 @@
 
 #include <linux/kernel.h>
 #include <linux/i2c.h>
-
+#include <linux/of.h>
 
 #include "exynos_drm_drv.h"
 #include "exynos_hdmi.h"