drm: Replace NULL with error value in drm_prime_pages_to_sg

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