drm/tilcdc: Fix non static symbol warning

Fixes the following sparse warning:

drivers/gpu/drm/tilcdc/tilcdc_drv.c:64:5: warning:
 symbol 'tilcdc_atomic_check' was not declared. Should it be static?
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Jyri Sarha <jsarha@ti.com>

drivers/gpu/drm/tilcdc/tilcdc_drv.c

@@ -75,8 +75,8 @@ static void tilcdc_fb_output_poll_changed(struct drm_device *dev)
 	drm_fbdev_cma_hotplug_event(priv->fbdev);
 }
 
-int tilcdc_atomic_check(struct drm_device *dev,
-			struct drm_atomic_state *state)
+static int tilcdc_atomic_check(struct drm_device *dev,
+			       struct drm_atomic_state *state)
 {
 	int ret;