drm/amd/display: checking for NULL instead of IS_ERR()

backlight_device_register() never returns NULL, it returns error
pointers on error so the check here is wrong.
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -1296,7 +1296,7 @@ amdgpu_dm_register_backlight_device(struct amdgpu_display_manager *dm)
 			&amdgpu_dm_backlight_ops,
 			&props);
 
-	if (NULL == dm->backlight_dev)
+	if (IS_ERR(dm->backlight_dev))
 		DRM_ERROR("DM: Backlight registration failed!\n");
 	else
 		DRM_DEBUG_DRIVER("DM: Registered Backlight device: %s\n", bl_name);