glx: Fix previous context validation in xorgGlxMakeCurrent

vnd has already verified that the context tag is valid before this gets called, and we only set the context tag private data to non-null for indirect clients. Mesa happens to be buggy and doesn't send MakeCurrent requests nearly as much as it should for direct contexts, but if you fix that, then unbinding a direct context would fail here with GLXBadContextTag.

Sadly Mesa will still need to carry a workaround here for broken servers, but we should still fix the server.