drm/exynos: fix return value check in lowlevel_buffer_allocate()

In case of error, the function drm_prime_pages_to_sg() returns ERR_PTR()
and never returns NULL. The NULL test in the return value check should
be replaced with IS_ERR().

Signed-off-by: Wei Yongjun <[e-mail]>
Signed-off-by: Inki Dae <[e-mail]>

drivers/gpu/drm/exynos/exynos_drm_buf.c

···
 	}
 
 	buf->sgt = drm_prime_pages_to_sg(buf->pages, nr_pages);
-	if (!buf->sgt) {
+	if (IS_ERR(buf->sgt)) {
 		DRM_ERROR("failed to get sg table.\n");
-		ret = -ENOMEM;
+		ret = PTR_ERR(buf->sgt);
 		goto err_free_attrs;
 	}