vfio/igd: Only emulate GGC register when x-igd-gms is set

vfio/igd: Only emulate GGC register when x-igd-gms is set

x-igd-gms is used for overriding DSM region size in GGC register in
both config space and MMIO BAR0, by default host value is used.
There is no need to emulate it in default case.

Signed-off-by: Tomita Moeko <tomitamoeko@gmail.com>
Reviewed-by: Corvin Köhne <c.koehne@beckhoff.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Tested-by: Alex Williamson <alex.williamson@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250505170305.23622-9-tomitamoeko@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>