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

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>