drm: Replace NULL with error value in drm_prime_pages_to_sg

Dan Carpenter has reported that there is the following static checker
warning:
	drivers/gpu/drm/drm_prime.c:317 drm_gem_map_dma_buf()
	warn: 'sgt' can also be NULL

314	sgt = obj->dev->driver->gem_prime_get_sg_table(obj);
315
316	if (!IS_ERR(sgt)) {
317		if (!dma_map_sg_attrs(attach->dev, sgt->sgl, sgt->nents, dir,

Problematic functions are xen_drm_front_gem_get_sg_table and
drm_gem_cma_prime_get_sg_table. Fix those by replacing NULL with error value.
Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180719093713.3643-1-andr2000@gmail.com

drivers/gpu/drm/drm_gem_cma_helper.c

@@ -436,7 +436,7 @@ struct sg_table *drm_gem_cma_prime_get_sg_table(struct drm_gem_object *obj)
 
 	sgt = kzalloc(sizeof(*sgt), GFP_KERNEL);
 	if (!sgt)
-		return NULL;
+		return ERR_PTR(-ENOMEM);
 
 	ret = dma_get_sgtable(obj->dev->dev, sgt, cma_obj->vaddr,
 			      cma_obj->paddr, obj->size);
@@ -447,7 +447,7 @@ struct sg_table *drm_gem_cma_prime_get_sg_table(struct drm_gem_object *obj)
 
 out:
 	kfree(sgt);
-	return NULL;
+	return ERR_PTR(ret);
 }
 EXPORT_SYMBOL_GPL(drm_gem_cma_prime_get_sg_table);
 

drivers/gpu/drm/xen/xen_drm_front_gem.c

@@ -179,7 +179,7 @@ struct sg_table *xen_drm_front_gem_get_sg_table(struct drm_gem_object *gem_obj)
 	struct xen_gem_object *xen_obj = to_xen_gem_obj(gem_obj);
 
 	if (!xen_obj->pages)
-		return NULL;
+		return ERR_PTR(-ENOMEM);
 
 	return drm_prime_pages_to_sg(xen_obj->pages, xen_obj->num_pages);
 }