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>
diff --git a/drivers/gpu/drm/nouveau/nouveau_display.c b/drivers/gpu/drm/nouveau/nouveau_display.c
index 74e6b4e..2e11fd6 100644
--- a/drivers/gpu/drm/nouveau/nouveau_display.c
+++ b/drivers/gpu/drm/nouveau/nouveau_display.c
@@ -84,16 +84,16 @@
 
 	gem = drm_gem_object_lookup(dev, file_priv, mode_cmd->handle);
 	if (!gem)
-		return NULL;
+		return ERR_PTR(-ENOENT);
 
 	nouveau_fb = kzalloc(sizeof(struct nouveau_framebuffer), GFP_KERNEL);
 	if (!nouveau_fb)
-		return NULL;
+		return ERR_PTR(-ENOMEM);
 
 	ret = nouveau_framebuffer_init(dev, nouveau_fb, mode_cmd, nouveau_gem_object(gem));
 	if (ret) {
 		drm_gem_object_unreference(gem);
-		return NULL;
+		return ERR_PTR(ret);
 	}
 
 	return &nouveau_fb->base;