vfio/igd: correctly calculate stolen memory size for gen 9 and later

vfio/igd: correctly calculate stolen memory size for gen 9 and later

We have to update the calculation of the stolen memory size because
we've seen devices using values of 0xf0 and above for the graphics mode
select field. The new calculation was taken from the linux kernel [1].

[1] https://github.com/torvalds/linux/blob/7c626ce4bae1ac14f60076d00eafe71af30450ba/arch/x86/kernel/early-quirks.c#L455-L460

Signed-off-by: Corvin Köhne <c.koehne@beckhoff.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>