drm/mediatek: dsi: Cleanup functions mtk_dsi_ps_control{_vact}()

Function mtk_dsi_ps_control() is a subset of mtk_dsi_ps_control_vact():
merge the two in one mtk_dsi_ps_control() function by adding one
function parameter `config_vact` which, when true, writes the VACT
related registers.

Reviewed-by: Fei Shao <fshao@chromium.org>
Reviewed-by: Alexandre Mergnat <amergnat@baylibre.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20240215085316.56835-4-angelogioacchino.delregno@collabora.com/


Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>