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>