drm/mgag200: Acquire I/O lock while reading EDID

DDC operation conflicts with concurrent mode setting. Acquire the
driver's I/O lock in get_modes to prevent this. This change should
have been part of commit 931e3f3a ("drm/mgag200: Protect
concurrent access to I/O registers with lock"), but apparently got
lost somewhere.

v3:
	* fix commit message to say 'drm/mgag200' (Jocelyn)
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: 931e3f3a ("drm/mgag200: Protect concurrent access to I/O registers with lock")
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Tested-by: Jocelyn Falempe <jfalempe@redhat.com>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Jocelyn Falempe <jfalempe@redhat.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Dave Airlie <airlied@redhat.com>
Cc: dri-devel@lists.freedesktop.org
Link: https://patchwork.freedesktop.org/patch/msgid/20220516134343.6085-2-tzimmermann@suse.de

drivers/gpu/drm/mgag200/mgag200_mode.c

@@ -691,16 +691,26 @@ static void mgag200_disable_display(struct mga_device *mdev)
 
 static int mga_vga_get_modes(struct drm_connector *connector)
 {
+	struct mga_device *mdev = to_mga_device(connector->dev);
 	struct mga_connector *mga_connector = to_mga_connector(connector);
 	struct edid *edid;
 	int ret = 0;
 
+	/*
+	 * Protect access to I/O registers from concurrent modesetting
+	 * by acquiring the I/O-register lock.
+	 */
+	mutex_lock(&mdev->rmmio_lock);
+
 	edid = drm_get_edid(connector, &mga_connector->i2c->adapter);
 	if (edid) {
 		drm_connector_update_edid_property(connector, edid);
 		ret = drm_add_edid_modes(connector, edid);
 		kfree(edid);
 	}
+
+	mutex_unlock(&mdev->rmmio_lock);
+
 	return ret;
 }