vfio/igd: align generation with i915 kernel driver

vfio/igd: align generation with i915 kernel driver

Define the igd device generations according to i915 kernel driver to
avoid confusion, and adjust comment placement to clearly reflect the
relationship between ids and devices.

The condition of how GTT stolen memory size is calculated is changed
accordingly as GGMS is in multiple of 2 starting from gen 8.

Reviewed-by: Corvin Köhne <c.koehne@beckhoff.com>
Signed-off-by: Tomita Moeko <tomitamoeko@gmail.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Link: https://lore.kernel.org/r/20241206122749.9893-4-tomitamoeko@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>