drm/sprd: fix potential NULL dereference

'drm' could be null in sprd_drm_shutdown, and drm_warn maybe dereference
it, remove this warning log.
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kevin Tang <kevin3.tang@gmail.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://lore.kernel.org/all/20220117084044.9210-1-kevin3.tang@gmail.com

v1 -> v2:
- Split checking platform_get_resource() return value to a separate patch
- Use dev_warn() instead of removing the warning log

drivers/gpu/drm/sprd/sprd_drm.c

@@ -154,7 +154,7 @@ static void sprd_drm_shutdown(struct platform_device *pdev)
 	struct drm_device *drm = platform_get_drvdata(pdev);
 
 	if (!drm) {
-		drm_warn(drm, "drm device is not available, no shutdown\n");
+		dev_warn(&pdev->dev, "drm device is not available, no shutdown\n");
 		return;
 	}