drm/mediatek: mtk_cec: Fix non-static global variable

drm/mediatek: mtk_cec: Fix non-static global variable

The struct 'mtk_cec_driver' is not used outside of the
mtk_cec.c file, so make it static to silence sparse warning:
```
drivers/gpu/drm/mediatek/mtk_cec.c:243:24: sparse: warning: symbol
'mtk_cec_driver' was not declared. Should it be static?
```

Fixes: 1e914a89ab7e ("drm/mediatek: mtk_cec: Switch to register as module_platform_driver")
Signed-off-by: Louis-Alexis Eyraud <louisalexis.eyraud@collabora.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20260429-mediatek-drm-fix-sparse-warnings-v1-3-d95c4d118b83@collabora.com/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>