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.
3 jobs for glx-make-current-fix in 12 minutes and 24 seconds (queued for 7 seconds)
Status Job ID Name Coverage
  Docker Image
passed #515161
debian-testing

00:00:38

 
  Build And Test
passed #515162
autotools-build-and-test

00:11:46

passed #515163
meson-build-and-test

00:04:08