drm: exynos: removed warning due to missing typecast for mixer driver data

Removing the warning by adding proper type casting where local pointer
variable of type mixer driver data is assigned with void pointer.
Signed-off-by: NRahul Sharma <rahul.sharma@samsung.com>
Signed-off-by: NInki Dae <inki.dae@samsung.com>
Signed-off-by: NKyungmin Park <kyungmin.park@samsung.com>

drivers/gpu/drm/exynos/exynos_mixer.c

@@ -1142,7 +1142,7 @@ static int __devinit mixer_probe(struct platform_device *pdev)
 		const struct of_device_id *match;
 		match = of_match_node(of_match_ptr(mixer_match_types),
 							  pdev->dev.of_node);
-		drv = match->data;
+		drv = (struct mixer_drv_data *)match->data;
 	} else {
 		drv = (struct mixer_drv_data *)
 			platform_get_device_id(pdev)->driver_data;