add self.CP = CP as with other carstate files (#459)

selfdrive/car/gm/carstate.py

@@ -51,6 +51,7 @@ def get_powertrain_can_parser(CP, canbus):
 
 class CarState(object):
   def __init__(self, CP, canbus):
+    self.CP = CP
     # initialize can parser
 
     self.car_fingerprint = CP.carFingerprint