drm/sun4i: dsi: Get the mod clock for A31

As per the user manual, look like mod clock is not mandatory
for all Allwinner MIPI DSI controllers, it is connected to
CLK_DSI_SCLK for A31 and not available in A64.

So, add compatible check for A31 and get mod clock accordingly.
Tested-by: Merlijn Wajer <merlijn@wizzup.org>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20191222132229.30276-4-jagan@amarulasolutions.com

drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c

@@ -1120,11 +1120,14 @@ static int sun6i_dsi_probe(struct platform_device *pdev)
 		return PTR_ERR(dsi->reset);
 	}
 
+	if (of_device_is_compatible(dev->of_node,
+				    "allwinner,sun6i-a31-mipi-dsi")) {
 		dsi->mod_clk = devm_clk_get(dev, "mod");
 		if (IS_ERR(dsi->mod_clk)) {
 			dev_err(dev, "Couldn't get the DSI mod clock\n");
 			return PTR_ERR(dsi->mod_clk);
 		}
+	}
 
 	/*
 	 * In order to operate properly, that clock seems to be always