drm: remove duplicate check on parent and avoid BUG_ON

In drm_dev_init, parent is checked for NULL via assert after
checked in devm_drm_dev_init(). The patch removes the duplicate
check and replaces the assertion with WARN_ON. Further, it returns
-EINVAL consistent with the usage in devm_drm_dev_init.
Signed-off-by: Aditya Pakki <pakki001@umn.edu>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20191215194345.4679-1-pakki001@umn.edu

drivers/gpu/drm/drm_drv.c

@@ -622,7 +622,8 @@ int drm_dev_init(struct drm_device *dev,
 		return -ENODEV;
 	}
 
-	BUG_ON(!parent);
+	if (WARN_ON(!parent))
+		return -EINVAL;
 
 	kref_init(&dev->ref);
 	dev->dev = get_device(parent);
@@ -725,7 +726,7 @@ int devm_drm_dev_init(struct device *parent,
 {
 	int ret;
 
-	if (WARN_ON(!parent || !driver->release))
+	if (WARN_ON(!driver->release))
 		return -EINVAL;
 
 	ret = drm_dev_init(dev, driver, parent);