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>

drivers/gpu/drm/exynos/exynos_mixer.c

@@ -30,6 +30,7 @@
 #include <linux/pm_runtime.h>
 #include <linux/clk.h>
 #include <linux/regulator/consumer.h>
+#include <linux/of.h>
 
 #include <drm/exynos_drm.h>