drm/mediatek: dp: Remove unused register definitions

Some definitions in mtk_dp_reg.h are not used, so remove these
redundant codes.

Signed-off-by: Bo-Chen Chen <rex-bc.chen@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Acked-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220916133821.27980-3-rex-bc.chen@mediatek.com