vfio/igd: fix GTT stolen memory size calculation for gen 8+

vfio/igd: fix GTT stolen memory size calculation for gen 8+

On gen 8 and later devices, the GTT stolen memory size when GGMS equals
0 is 0 (no preallocated memory) rather than 1MB [1].

[1] 3.1.13, 5th Generation Intel Core Processor Family Datasheet Vol. 2
    https://www.intel.com/content/www/us/en/content-details/330835

Fixes: c4c45e943e51 ("vfio/pci: Intel graphics legacy mode assignment")
Reported-By: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Tomita Moeko <tomitamoeko@gmail.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Link: https://lore.kernel.org/r/20241206122749.9893-2-tomitamoeko@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>