vfio/igd: canonicalize memory size calculations

vfio/igd: canonicalize memory size calculations

Add helper functions igd_gtt_memory_size() and igd_stolen_size() for
calculating GTT stolen memory and Data stolen memory size in bytes,
and use macros to replace the hardware-related magic numbers for
better readability.

Signed-off-by: Tomita Moeko <tomitamoeko@gmail.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Link: https://lore.kernel.org/r/20241206122749.9893-5-tomitamoeko@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>