drm/mediatek: gamma: Add kerneldoc for struct mtk_disp_gamma

The mtk_disp_gamma structure was completely undocumented: add some
kerneldoc documentation to it.

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/20231012095736.100784-16-angelogioacchino.delregno@collabora.com/
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>

Author: AngeloGioacchino Del Regno

drivers/gpu/drm/mediatek/mtk_disp_gamma.c

@@ -50,8 +50,12 @@ struct mtk_disp_gamma_data {
 	u8 lut_bits;
 };
 
-/*
- * struct mtk_disp_gamma - DISP_GAMMA driver structure
+/**
+ * struct mtk_disp_gamma - Display Gamma driver structure
+ * @clk:      clock for DISP_GAMMA block
+ * @regs:     MMIO registers base
+ * @cmdq_reg: CMDQ Client register
+ * @data:     platform data for DISP_GAMMA
  */
 struct mtk_disp_gamma {
 	struct clk *clk;