gallium/util: fix memory-leak

When I originally wrote this code, I forgot to release the views the
code creates, leaking a bit of memory that never gets cleaned up. That's
not great, so let's plug it.

Fixes: e8a40715a8b ("gallium/util: add blitter-support for stencil-fallback")
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6960>
diff --git a/src/gallium/auxiliary/util/u_blitter.c b/src/gallium/auxiliary/util/u_blitter.c
index ddb0065..1de3f75 100644
--- a/src/gallium/auxiliary/util/u_blitter.c
+++ b/src/gallium/auxiliary/util/u_blitter.c
@@ -2911,4 +2911,7 @@
    util_blitter_restore_render_cond(blitter);
    util_blitter_restore_constant_buffer_state(blitter);
    util_blitter_unset_running_flag(blitter);
+
+   pipe_surface_reference(&dst_view, NULL);
+   pipe_sampler_view_reference(&src_view, NULL);
 }