usb: typec: ucsi: displayport: Fix NULL pointer dereference

commit d16e7b62 upstream.

If the registration of the DisplayPort was not successful,
or if the port does not support DisplayPort alt mode in the
first place, the function ucsi_displayport_remove_partner()
will fail with NULL pointer dereference when it attempts to
access the driver data.

Adding a check to the function to make sure there really is
driver data for the device before modifying it.

Fixes: af8622f6

 ("usb: typec: ucsi: Support for DisplayPort alt mode")
Reported-by: Andrea Gagliardi La Gala <andrea.lagala@gmail.com>
BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=206365
Cc: stable@vger.kernel.org
Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Link: https://lore.kernel.org/r/20200311130006.41288-2-heikki.krogerus@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>