compositor-drm: Remember to drop master in drm error handling

Otherwise we'll kill whatever other display sever we're switching back to.
The tricky thing here is that we never explicitly set drm master in the
startup path, we get that implicitly from being the first to open the
drm device.  Even so, we need to drop it before switching VTs.

src/compositor-drm.c

@@ -2376,6 +2376,8 @@ err_sprite:
 err_udev_dev:
 	udev_device_unref(drm_device);
 err_tty:
+	if (weston_launcher_drm_set_master(&ec->base, ec->drm.fd, 0) < 0)
+		weston_log("failed to drop master: %m\n");
 	tty_destroy(ec->tty);
 err_udev:
 	udev_unref(ec->udev);