drm/mediatek: dsi: Use symbolized register definition

For HSTX_CKLP_EN and DIS_EOT, use symbolized register
definition instead of magic number.

Signed-off-by: Jitao Shi <jitao.shi@mediatek.com>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>