drm/exynos: return NULL if exynos_pages_to_sg fails

exynos_pages_to_sg() internally calls sg_kmalloc() which can return
no pages when the system is under high memory crunch. One such instance
is chromeos-install in the chromeos. This patch adds check for the return
value of the function in subject to return NULL on failure.
Signed-off-by: NSubash Patel <subash.ramaswamy@linaro.org>
Signed-off-by: NInki Dae <inki.dae@samsung.com>
Signed-off-by: NKyungmin Park <kyungmin.park@samsung.com>

drivers/gpu/drm/exynos/exynos_drm_dmabuf.c

@@ -86,6 +86,10 @@ static struct sg_table *
 	npages = buf->size / buf->page_size;
 
 	sgt = exynos_pages_to_sg(buf->pages, npages, buf->page_size);
+	if (!sgt) {
+		DRM_DEBUG_PRIME("exynos_pages_to_sg returned NULL!\n");
+		goto err_unlock;
+	}
 	nents = dma_map_sg(attach->dev, sgt->sgl, sgt->nents, dir);
 
 	DRM_DEBUG_PRIME("npages = %d buffer size = 0x%lx page_size = 0x%lx\n",