drm/bridge: dumb-vga-dac: Fix dereferencing -ENODEV DDC channel

If the VGA connector has no DDC channel, an error pointer will be
dereferenced, e.g. on Salvator-XS:

    Unable to handle kernel NULL pointer dereference at virtual address 000000000000017d
    ...
    Call trace:
     sysfs_do_create_link_sd.isra.0+0x40/0x108
     sysfs_create_link+0x20/0x40
     drm_sysfs_connector_add+0xa8/0xc8
     drm_connector_register.part.3+0x54/0xb0
     drm_connector_register_all+0xb0/0xd0
     drm_modeset_register_all+0x54/0x88
     drm_dev_register+0x18c/0x1d8
     rcar_du_probe+0xe4/0x150
     ...

This happens because vga->ddc either contains a valid DDC channel
pointer, or -ENODEV, and drm_connector_init_with_ddc() expects a valid
DDC channel pointer, or NULL.

Fix this by resetting vga->ddc to NULL in case of -ENODEV, and replacing
the existing error checks by non-NULL checks.
This is similar to what the HDMI connector driver does.

Fixes: a4f9087e ("drm/bridge: dumb-vga-dac: Provide ddc symlink in connector sysfs directory")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190813093046.4976-1-geert+renesas@glider.be

drivers/gpu/drm/bridge/dumb-vga-dac.c

@@ -42,7 +42,7 @@ static int dumb_vga_get_modes(struct drm_connector *connector)
 	struct edid *edid;
 	int ret;
 
-	if (IS_ERR(vga->ddc))
+	if (!vga->ddc)
 		goto fallback;
 
 	edid = drm_get_edid(connector, vga->ddc);
@@ -84,7 +84,7 @@ dumb_vga_connector_detect(struct drm_connector *connector, bool force)
 	 * wire the DDC pins, or the I2C bus might not be working at
 	 * all.
 	 */
-	if (!IS_ERR(vga->ddc) && drm_probe_ddc(vga->ddc))
+	if (vga->ddc && drm_probe_ddc(vga->ddc))
 		return connector_status_connected;
 
 	return connector_status_unknown;
@@ -197,6 +197,7 @@ static int dumb_vga_probe(struct platform_device *pdev)
 		if (PTR_ERR(vga->ddc) == -ENODEV) {
 			dev_dbg(&pdev->dev,
 				"No i2c bus specified. Disabling EDID readout\n");
+			vga->ddc = NULL;
 		} else {
 			dev_err(&pdev->dev, "Couldn't retrieve i2c bus\n");
 			return PTR_ERR(vga->ddc);
@@ -218,7 +219,7 @@ static int dumb_vga_remove(struct platform_device *pdev)
 
 	drm_bridge_remove(&vga->bridge);
 
-	if (!IS_ERR(vga->ddc))
+	if (vga->ddc)
 		i2c_put_adapter(vga->ddc);
 
 	return 0;