i915c: Add a symlink for i830_dri.so
The gallium driver doesn't support gen2, so let's make it possible to keep both i915g and i830 drivers installed in parallel.
This wants a corresponding xserver change, but this at least lays the groundwork.
I'm also assuming here that the LTS branch will continue to build an i915 driver until/unless we're confident that i915g is a superior replacement. As a result the classic build still claims the i915_dri.so symlink, we can revisit that when/if we decide to switch LTS to gen2-only.