drm/exynos: mixer: Constify platform_device_id

The platform_device_id is not modified by the driver and core uses it as
const.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski.k@gmail.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>

drivers/gpu/drm/exynos/exynos_mixer.c

@@ -1159,7 +1159,7 @@ static struct mixer_drv_data exynos4210_mxr_drv_data = {
 	.has_sclk = 1,
 };
 
-static struct platform_device_id mixer_driver_types[] = {
+static const struct platform_device_id mixer_driver_types[] = {
 	{
 		.name		= "s5p-mixer",
 		.driver_data	= (unsigned long)&exynos4210_mxr_drv_data,