gallium/u_transfer_helper: Don't leak a reference to the resource.

We pipe_resource_reference when handling transfers in map, we need to
do a corresponding unreference in unmap.

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
diff --git a/src/gallium/auxiliary/util/u_transfer_helper.c b/src/gallium/auxiliary/util/u_transfer_helper.c
index b7bd43f..033a49b 100644
--- a/src/gallium/auxiliary/util/u_transfer_helper.c
+++ b/src/gallium/auxiliary/util/u_transfer_helper.c
@@ -511,6 +511,8 @@
             helper->vtbl->transfer_unmap(pctx, trans->trans2);
       }
 
+      pipe_resource_reference(&ptrans->resource, NULL);
+
       free(trans->staging);
       free(trans);
    } else {