configure.ac: correct the xlib/xlib-gallium GLX detection for GLVND

Things have changed since commit a92910a ("glx: Refactor the configure
options for glx implementation choice (v3)") where only a single
configure option is used to control the GLX provider.

[Emil Velikov: Ensure that the check is moved after the detection code.]
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
1 file changed