vfio/igd: Fix potential overflow in igd_gtt_memory_size()

vfio/igd: Fix potential overflow in igd_gtt_memory_size()

The risk is mainly theoretical since the applied bit mask will keep
the 'ggms' shift value below 3. Nevertheless, let's use a 64 bit
integer type and resolve the coverity issue.

Resolves: Coverity CID 1585908
Fixes: 1e1eac5f3dcd ("vfio/igd: canonicalize memory size calculations")
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Link: https://lore.kernel.org/r/20250107130604.669697-1-clg@redhat.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>