radeon_display: Use pointer return error codes

drm_mode_addfb() expects fb_create return error code
instead of NULL.
Signed-off-by: Nxueminsu <xuemin.su@intel.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org

drivers/gpu/drm/radeon/radeon_display.c

@@ -1122,7 +1122,7 @@ radeon_user_framebuffer_create(struct drm_device *dev,
 	if (ret) {
 		kfree(radeon_fb);
 		drm_gem_object_unreference_unlocked(obj);
-		return NULL;
+		return ERR_PTR(ret);
 	}
 
 	return &radeon_fb->base;