drm/mediatek: gamma: Enable the Gamma LUT table only after programming

Move the write to DISP_GAMMA_CFG to enable the Gamma LUT to after
programming the actual table to avoid potential visual glitches during
table modification.

Note:
GAMMA should get enabled in between vblanks, but this requires many
efforts in order to make this happen, as that requires migrating all
of the writes to make use of CMDQ instead of cpu writes and that's
not trivial. For this reason, this patch only moves the LUT enable.
The CMDQ rework will come at a later time.

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/20231012095736.100784-6-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

@@ -65,12 +65,12 @@ unsigned int mtk_gamma_get_lut_size(struct device *dev)
 void mtk_gamma_set_common(struct device *dev, void __iomem *regs, struct drm_crtc_state *state)
 {
 	struct mtk_disp_gamma *gamma;
-	unsigned int i, reg;
+	unsigned int i;
 	struct drm_color_lut *lut;
 	void __iomem *lut_base;
 	bool lut_diff;
 	u16 lut_size;
-	u32 word;
+	u32 cfg_val, word;
 
 	/* If there's no gamma lut there's nothing to do here. */
 	if (!state->gamma_lut)
@@ -87,9 +87,6 @@ void mtk_gamma_set_common(struct device *dev, void __iomem *regs, struct drm_crt
 		lut_size = 512;
 	}
 
-	reg = readl(regs + DISP_GAMMA_CFG);
-	reg = reg | GAMMA_LUT_EN;
-	writel(reg, regs + DISP_GAMMA_CFG);
 	lut_base = regs + DISP_GAMMA_LUT;
 	lut = (struct drm_color_lut *)state->gamma_lut->data;
 	for (i = 0; i < lut_size; i++) {
@@ -119,6 +116,13 @@ void mtk_gamma_set_common(struct device *dev, void __iomem *regs, struct drm_crt
 		}
 		writel(word, lut_base + i * 4);
 	}
+
+	cfg_val = readl(regs + DISP_GAMMA_CFG);
+
+	/* Enable the gamma table */
+	cfg_val |= GAMMA_LUT_EN;
+
+	writel(cfg_val, regs + DISP_GAMMA_CFG);
 }
 
 void mtk_gamma_set(struct device *dev, struct drm_crtc_state *state)