drm/tegra: Fix color expansion

bpp stores the number of bytes per pixel, but color expansion needs to
be enabled for less than 24 bits per pixel.
Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>

drivers/gpu/drm/tegra/dc.c

@@ -500,7 +500,7 @@ int tegra_dc_setup_window(struct tegra_dc *dc, unsigned int index,
 		tegra_dc_writel(dc, 0x0000, DC_WIN_CSC_KVB);
 
 		value |= CSC_ENABLE;
-	} else if (bpp < 24) {
+	} else if (window->bits_per_pixel < 24) {
 		value |= COLOR_EXPAND;
 	}