drm/mediatek: mtk_disp_gamma: Fix breakage due to merge issue

drm/mediatek: mtk_disp_gamma: Fix breakage due to merge issue

While the commit that was sent to the mailing lists was fine, something
happened during merge and the mtk_gamma_set() function got broken as
a writel() was turned into a readl().

Fix that by changing that back to the expected writel().

Fixes: a6b39cd248f3 ("drm/mediatek: De-commonize disp_aal/disp_gamma gamma_set functions")
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/20231025104940.140605-1-angelogioacchino.delregno@collabora.com/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>