drm: Propagate error code from fb_create()

Change the interface to expect a PTR_ERR specifing the real error code
as opposed to assuming a NULL return => -EINVAL. Just once the user may
not be at fault!

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_crtc.c

@@ -1682,9 +1682,9 @@ int drm_mode_addfb(struct drm_device *dev,
 	/* TODO setup destructor callback */
 
 	fb = dev->mode_config.funcs->fb_create(dev, file_priv, r);
-	if (!fb) {
+	if (IS_ERR(fb)) {
 		DRM_ERROR("could not create framebuffer\n");
-		ret = -EINVAL;
+		ret = PTR_ERR(fb);
 		goto out;
 	}