x86/AMD-Vi: Fix IVRS HPET special->handle override

x86/AMD-Vi: Fix IVRS HPET special->handle override

The current logic does not handle the case when HPET special->handle
is invalid in IVRS. On such system, the following message is shown:

(XEN) AMD-Vi: Failed to setup HPET MSI remapping: Wrong HPET

This patch will allow the ivrs_hpet[<handle>]=<sbdf> to override the
IVRS.  Also, it removes struct hpet_sbdf.iommu since it is not
used anywhere in the code.

Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>