glx: Remove some excess work from the GLX_FBCONFIG_ID fallback
The config matched by visual ID will have the right fbconfig ID set, you don't need to go looking for it, so long as you look in the list of fbconfigs as opposed to the list of visuals (which do not have GLX_FBCONFIG_ID filled in).
It's sort of broken that we have two lists here, when the only real distinction is that glXChooseVisuals needs to filter out non-window- capable configs from its view. That's a bigger cleanup for another day.
Reviewed-by: @anholt (for a previous but broken version, this one makes piglit happy though)