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

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