intel-iommu: Use symbolic values instead of magic numbers in Lenovo w/a

Commit 9eecabcb9a924f1e11ba670365fd4babe423045c ("intel-iommu: Abort
IOMMU setup for igfx if BIOS gave no shadow GTT space") uses a bunch of
magic numbers. Provide #defines for those to make it look slightly saner.

Signed-off-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>