Fix this sparse warning:
  drivers/gpu/drm/radeon/r600_cp.c:1811:52: warning: Using plain integer as NULL pointer
  drivers/gpu/drm/radeon/radeon_cp.c:1363:52: warning: Using plain integer as NULL pointer
  drivers/gpu/drm/radeon/radeon_state.c:1983:61: warning: Using plain integer as NULL pointer
Signed-off-by: NHannes Eder <hannes@hanneseder.net>
Signed-off-by: NDave Airlie <airlied@redhat.com>