drm/radeon: NULL noise: drivers/gpu/drm/radeon/radeon_*.c

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: Hannes Eder <hannes@hanneseder.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>
diff --git a/drivers/gpu/drm/radeon/r600_cp.c b/drivers/gpu/drm/radeon/r600_cp.c
index be2bba6..6f2cc74 100644
--- a/drivers/gpu/drm/radeon/r600_cp.c
+++ b/drivers/gpu/drm/radeon/r600_cp.c
@@ -1811,7 +1811,7 @@
 
 		if (dev_priv->gart_info.gart_table_location == DRM_ATI_GART_FB) {
 			drm_core_ioremapfree(&dev_priv->gart_info.mapping, dev);
-			dev_priv->gart_info.addr = 0;
+			dev_priv->gart_info.addr = NULL;
 		}
 	}
 	/* only clear to the start of flags */