drm/mediatek: Modify dsi funcs to atomic operations

Because .enable & .disable are deprecated.
Use .atomic_enable & .atomic_disable instead.

Link: https://patchwork.kernel.org/project/linux-mediatek/patch/1653012007-11854-2-git-send-email-xinlei.lee@mediatek.com/
Signed-off-by: Jitao Shi <jitao.shi@mediatek.com>
Signed-off-by: Xinlei Lee <xinlei.lee@mediatek.com>
Reviewed-by: Rex-BC Chen <rex-bc.chen@mediatek.com>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>