drm: Fix sparce warning in radeon driver

From: Luiz Fernando Capitulino <[e-mail]>

drivers/char/drm/radeon_cp.c:1643:31: warning: Using plain integer as NULL
pointer

Signed-off-by: Luiz Capitulino <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Dave Airlie <[e-mail]>

drivers/char/drm/radeon_cp.c

···
 		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 */