drm/mediatek: mtk_hdmi_v2: Fix non-static global variable

drm/mediatek: mtk_hdmi_v2: Fix non-static global variable

The struct 'mtk_hdmi_v2_clk_names' is not used outside of the
mtk_hdmi_v2.c file, so make it static to silence sparse warning:
```
drivers/gpu/drm/mediatek/mtk_hdmi_v2.c:53:12: sparse: warning: symbol
'mtk_hdmi_v2_clk_names' was not declared. Should it be static?
```

Fixes: 8d0f79886273 ("drm/mediatek: Introduce HDMI/DDC v2 for MT8195/MT8188")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202604132044.fcYjEcU8-lkp@intel.com/
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-v1-2-d95c4d118b83@collabora.com/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>