glx: GLX_MESA_multithread_makecurrent is direct-only

glx: GLX_MESA_multithread_makecurrent is direct-only

This extension is not defined for indirect contexts. Marking it as
"client only", as the old code did here, would make the extension
available in indirect contexts, even though the server would certainly
not have it in its extension list.

Cc: <mesa-stable@lists.freedesktop.org>
Signed-off-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>