drm/mediatek: dsi: Use bitfield macros where useful

Instead of open coding bitshifting for various register fields,
use the bitfield macro FIELD_PREP(): this allows to enhance the
human readability, decrease likeliness of mistakes (and register
field overflowing) and also to simplify the code.
The latter is especially seen in mtk_dsi_rxtx_control(), where
it was possible to change a switch to a short for loop and to
also remove the need to check for maximum DSI lanes == 4 thanks
to the FIELD_PREP macro masking the value.

While at it, also add the missing DA_HS_SYNC bitmask, used in
mtk_dsi_phy_timconfig().

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/20240215085316.56835-5-angelogioacchino.delregno@collabora.com/


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