Probe for i830_dri.so and, if it exists, use that driver in preference
to i915_dri.so for DRI (i.e. GL clients). This was suggested in the
context of distributions supplying i915g as the main DRI driver for
gen3, in which case we need to provide a separate DRI driver for gen2.
Signed-off-by: Chris Wilson <email@example.com>