drm/exynos: Fix potential NULL pointer dereference in suspend/resume paths

The patch 6e8edf8a: "drm/exynos: Fix suspend/resume support" introduced
a new code in suspend/resume paths. However it unconditionally dereference
drm_dev pointer, which might be NULL if suspend/resume happens before
Exynos DRM driver components bind. This patch fixes this issue.
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 6e8edf8a "drm/exynos: Fix suspend/resume support"
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>

drivers/gpu/drm/exynos/exynos_drm_drv.c

@@ -168,11 +168,13 @@ static struct drm_driver exynos_drm_driver = {
 static int exynos_drm_suspend(struct device *dev)
 {
 	struct drm_device *drm_dev = dev_get_drvdata(dev);
-	struct exynos_drm_private *private = drm_dev->dev_private;
+	struct exynos_drm_private *private;
 
 	if (pm_runtime_suspended(dev) || !drm_dev)
 		return 0;
 
+	private = drm_dev->dev_private;
+
 	drm_kms_helper_poll_disable(drm_dev);
 	exynos_drm_fbdev_suspend(drm_dev);
 	private->suspend_state = drm_atomic_helper_suspend(drm_dev);
@@ -188,11 +190,12 @@ static int exynos_drm_suspend(struct device *dev)
 static int exynos_drm_resume(struct device *dev)
 {
 	struct drm_device *drm_dev = dev_get_drvdata(dev);
-	struct exynos_drm_private *private = drm_dev->dev_private;
+	struct exynos_drm_private *private;
 
 	if (pm_runtime_suspended(dev) || !drm_dev)
 		return 0;
 
+	private = drm_dev->dev_private;
 	drm_atomic_helper_resume(drm_dev, private->suspend_state);
 	exynos_drm_fbdev_resume(drm_dev);
 	drm_kms_helper_poll_enable(drm_dev);