Revert "vfio/pci-quirks.c: Disable stolen memory for igd VFIO"

This reverts commit c2b2e158.

The original patch intend to prevent linux i915 driver from using
stolen meory. But this patch breaks windows IGD driver loading on
Gen9+, as IGD HW will use stolen memory on Gen9+, once windows IGD
driver see zero size stolen memory, it will unload.
Meanwhile stolen memory will be disabled in 915 when i915 run as
a guest.

Signed-off-by: Xiong Zhang <xiong.y.zhang@intel.com>
[aw: Gen9+ is SkyLake and newer]
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>