drm/mediatek: mtk_dsi: Enable HS clock only at pre-enable

drm/mediatek: mtk_dsi: Enable HS clock only at pre-enable

Commit 76255024cadb ("drm/mediatek: mtk_dsi: enable hs clock
during pre-enable") rightfully moves the HS clock enablement to
before atomic_enable(), but it's moving it to mtk_dsi_poweron(),
which is not only called in the .atomic_pre_enable() callback
for the DRM bridge, but also in the MediaTek DRM's .ddp_start()
callback, which happens way before the bridge ones.

The HS clock enablement should be done at just the right time,
otherwise some bridge chips (or some Display Driver ICs) may
not work correctly: this is seen at least with a Parade DSI to
eDP bridge (PS8640) on the MT8173 Elm Chromebook.

This resolves a regression that was seen on the aforementioned
machine, which was happening only after a suspend-resume cycle.

Cc: <stable@vger.kernel.org> # 7.1.x
Fixes: 76255024cadb ("drm/mediatek: mtk_dsi: enable hs clock during pre-enable")
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Tested-by: Adam Thiede <me@adamthiede.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Link: https://patchwork.kernel.org/project/linux-mediatek/patch/20260728153145.578404-1-angelogioacchino.delregno@collabora.com/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>