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>
 
                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 */
 
                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 */
 
 
        /* find a virtual surface */
        for (i = 0; i < 2 * RADEON_MAX_SURFACES; i++)
-               if (dev_priv->virt_surfaces[i].file_priv == 0)
+               if (dev_priv->virt_surfaces[i].file_priv == NULL)
                        break;
        if (i == 2 * RADEON_MAX_SURFACES) {
                return -1;