drm/mediatek: gamma: Program gamma LUT type for descending or rising

All of the SoCs that don't have dithering control in the gamma IP
have got a GAMMA_LUT_TYPE bit that tells to the IP if the LUT is
"descending" (bit set) or "rising" (bit cleared): make sure to set
it correctly after programming the LUT.

Reviewed-by: Jason-JH.Lin <jason-jh.lin@mediatek.com>
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/20231012095736.100784-14-angelogioacchino.delregno@collabora.com/


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