drm/stm: ltdc: reset plane transparency after plane disable

The plane's opacity should be reseted while the plane
is disabled. It prevents from seeing a possible global
or layer background color set earlier.
Signed-off-by: Yannick Fertre <yannick.fertre@foss.st.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240712131344.98113-1-yannick.fertre@foss.st.comSigned-off-by: Raphael Gallais-Pou <raphael.gallais-pou@foss.st.com>

drivers/gpu/drm/stm/ltdc.c

@@ -1508,6 +1508,9 @@ static void ltdc_plane_atomic_disable(struct drm_plane *plane,
 	/* Disable layer */
 	regmap_write_bits(ldev->regmap, LTDC_L1CR + lofs, LXCR_LEN | LXCR_CLUTEN |  LXCR_HMEN, 0);
 
+	/* Reset the layer transparency to hide any related background color */
+	regmap_write_bits(ldev->regmap, LTDC_L1CACR + lofs, LXCACR_CONSTA, 0x00);
+
 	/* Commit shadow registers = update plane at next vblank */
 	if (ldev->caps.plane_reg_shadow)
 		regmap_write_bits(ldev->regmap, LTDC_L1RCR + lofs,