glx: Only clear the stored context tag when the context has been unbound

glx: Only clear the stored context tag when the context has been unbound

The calling order of ->bind and ->unbind changed and then ->unbind would
clear the currentContextTag of the old context before ->bind could reuse
it in the make current request, in the indirect case.

Instead, clear the old currentContextTag if and only if we send a request
to the server to actually unbind it or reassign it to another context.

https://bugs.freedesktop.org/show_bug.cgi?id=29977

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Signed-off-by: Kristian Høgsberg <krh@bitplanet.net>