drm/mediatek: mtk_dsi: Add support for High Speed (HS) mode

drm/mediatek: mtk_dsi: Add support for High Speed (HS) mode

Up until now, the MediaTek DSI Controller has always been working
in Low Power Mode (LPM), as this driver has always ignored the
MIPI_DSI_MSG_USE_LPM flag hence never setting HS mode.

In the current state of the driver the only thing that is needed
to add support for DSI High Speed (HS) transmit is to simply set
the "HSTX" config bit in the configuration register.

Check if flag MIPI_DSI_MSG_USE_LPM is set and, if not, set HSTX.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Tested-by: Gary Bisson <bisson.gary@gmail.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20260108101959.14872-1-angelogioacchino.delregno@collabora.com/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>