iommu/vt-d: Set intel_iommu_gfx_mapped correctly

The intel_iommu_gfx_mapped flag is exported by the Intel
IOMMU driver to indicate whether an IOMMU is used for the
graphic device. In a virtualized IOMMU environment (e.g.
QEMU), an include-all IOMMU is used for graphic device.
This flag is found to be clear even the IOMMU is used.

Cc: Ashok Raj <ashok.raj@intel.com>
Cc: Jacob Pan <jacob.jun.pan@linux.intel.com>
Cc: Kevin Tian <kevin.tian@intel.com>
Reported-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Fixes: c0771df8 ("intel-iommu: Export a flag indicating that the IOMMU is used for iGFX.")
Suggested-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>