drm/udl: Fix missing error code in udl_handle_damage()

If udl_get_urb() fails then this should return a negative error code
but currently it returns success.

Fixes: 798ce3fe ("drm/udl: Begin/end access to imported buffers in damage-handler")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20201113101502.GD168908@mwanda

drivers/gpu/drm/udl/udl_modeset.c

@@ -305,8 +305,10 @@ static int udl_handle_damage(struct drm_framebuffer *fb, int x, int y,
 	vaddr = map.vaddr; /* TODO: Use mapping abstraction properly */
 
 	urb = udl_get_urb(dev);
-	if (!urb)
+	if (!urb) {
+		ret = -ENOMEM;
 		goto out_drm_gem_shmem_vunmap;
+	}
 	cmd = urb->transfer_buffer;
 
 	for (i = clip.y1; i < clip.y2; i++) {