configure.ac: correct driglx-direct help text

The default was toggled a while back, but the text wasn't updated.

Fixes: bd526ec9 ("configure: Always default to
--enable-driglx-direct")
Cc: Jon TURNEY <jon.turney@dronecode.org.uk>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
(cherry picked from commit 6aeef546)