docs: promote #dri-devel on oftc over freenode

Due to the recent changes at Freenode and the recommendation from the X.Org board of moving to OFTC, many mesa-developers have moved over to the #dri-devel channel on OFTC by now.

So let's promote that channel rather than the Freenode channel.

We're doing the same thing for #zink and #panfrost, so let's update the docs for those drivers while we're at it.