mesa: fix-up and use _mesa_delete_renderbuffer()

mesa: fix-up and use _mesa_delete_renderbuffer()

_mesa_delete_renderbuffer() should free the mutex (though that may be a
no-op) and then free the renderbuffer object itself.  Subclasses of
gl_renderbuffer can use this function too.

Reviewed-by: José Fonseca <jfonseca@vmware.com>