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 <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Inki Dae <inki.dae@samsung.com>

drivers/gpu/drm/exynos/exynos_drm_buf.c

@@ -91,9 +91,9 @@ static int lowlevel_buffer_allocate(struct drm_device *dev,
 	}
 
 	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;
 	}