drm/mediatek: Fix potential memory leak if vmap() fail

Also return -ENOMEM if such a failure happens, the implement should take
responsibility for the error handling.

Fixes: 3df64d7b ("drm/mediatek: Implement gem prime vmap/vunmap function")
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Reviewed-by: Alexandre Mergnat <amergnat@baylibre.com>
Signed-off-by: Sui Jingfeng <suijingfeng@loongson.cn>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20230706134000.130098-1-suijingfeng@loongson.cn/Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>

drivers/gpu/drm/mediatek/mtk_drm_gem.c

@@ -247,7 +247,11 @@ int mtk_drm_gem_prime_vmap(struct drm_gem_object *obj, struct iosys_map *map)
 
 	mtk_gem->kvaddr = vmap(mtk_gem->pages, npages, VM_MAP,
 			       pgprot_writecombine(PAGE_KERNEL));
-
+	if (!mtk_gem->kvaddr) {
+		kfree(sgt);
+		kfree(mtk_gem->pages);
+		return -ENOMEM;
+	}
 out:
 	kfree(sgt);
 	iosys_map_set_vaddr(map, mtk_gem->kvaddr);