Create a way to prefer iris vs i965 via driconf
I asked @ajax about enabling iris by default, and he suggested that it would be nice to allow users to switch between the two via a driconf setting. This way, distributions could ship a snippet in /usr/share/drirc.d to select their preferred driver and users could edit that or override it in ~/.drirc in case they hit a snag and want to switch drivers. It seems like a decent idea.
You can almost do this today. driconf supports this snippet:
<driconf>
<device driver="loader" kernel_driver="i915">
<option name="dri_driver" value="iris" />
</device>
</driconf>
Unfortunately, this loads for all Intel hardware, even cards that iris doesn't support. We could perhaps update this mechanism to only override the driver among supported choices for the given PCI ID (i.e. ignore it if the value isn't in the driver list). Or extend it to allow matching only a subset of cards somehow.
Ideally we would then add a meson build option for the preferred driver, and install a /usr/share/drirc.d snippet with their preferred setting.