zink/gallium/dri2/gbm/glx/egl: allow drivers to choose to use zink instead.

Currently to use zink you have to set the loader override or hit in a fallback path. This isn't ideal. It would be good if a driver could signify that it would like that zink be used instead of it, depending on build flags or hardware versions.

For nouveau we'd like to move to using zink on nvk for newer GPUs, so this lays the ground work.

This is a second rewrite of this to have the loader make the decision by probing the kernel itself, it still needs fallback fixes in dri2/3 paths as we can't use those for loading zink, but it's a much smaller footprint