drm/mediatek: mtk_hdmi: Remove unused members of struct mtk_hdmi

drm/mediatek: mtk_hdmi: Remove unused members of struct mtk_hdmi

The hdmi_colorspace csp member of struct mtk_hdmi is initialized
once but then it's never used at all.
Remove said member and the only assignment to it as a cleanup.

Also remove the ibias, ibias_up, min_clock, max_clock, min_hdisplay
and max_vdisplay members, as those were really completely unused.

This commit brings no functional changes.

Reviewed-by: CK Hu <ck.hu@mediatek.com>
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://patchwork.kernel.org/project/linux-mediatek/patch/20250217154836.108895-28-angelogioacchino.delregno@collabora.com/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>