drm/mediatek: Drop unbalanced obj unref

In the error path, mtk_drm_gem_object_mmap() is dropping an obj
reference that it doesn't own.

Fixes: 119f5173 ("drm/mediatek: Add DRM Driver for Mediatek SoC MT8173.")
Signed-off-by: Rob Clark <robdclark@chromium.org>
Link: https://patchwork.kernel.org/project/linux-mediatek/patch/20230119231255.2883365-1-robdclark@gmail.com/Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>

drivers/gpu/drm/mediatek/mtk_drm_gem.c

@@ -169,8 +169,6 @@ static int mtk_drm_gem_object_mmap(struct drm_gem_object *obj,
 
 	ret = dma_mmap_attrs(priv->dma_dev, vma, mtk_gem->cookie,
 			     mtk_gem->dma_addr, obj->size, mtk_gem->dma_attrs);
-	if (ret)
-		drm_gem_vm_close(vma);
 
 	return ret;
 }