glx: Perform check for valid fbconfig against proper X-Screen.

glx: Perform check for valid fbconfig against proper X-Screen.

Commit cf804b4455fac9e585b3600a8318caaced9c23de
('glx: fix crash with bad fbconfig') introduced a check
in glXCreateNewContext() if the given config is a valid
fbconfig.

Unfortunately the check always checks the given config against
the fbconfigs of the DefaultScreen(dpy), instead of the
actual X-Screen specified in the config config->screen.

This leads to failure whenever a GL context is created
on a non-DefaultScreen(dpy), e.g., on X-Screen 1 of
a multi-x-screen setup, where the default screen is
typically 0.

Fix this by using config->screen instead of DefaultScreen(dpy).

Tested to fix context creation failure on a dual-x-screen setup.

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Cc: "11.2 12.0" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>