iommu/vt-d: Decouple igfx_off from graphic identity mapping

A kernel command called igfx_off was introduced in commit <ba395927>
("Intel IOMMU: Intel IOMMU driver"). This command allows the user to
disable the IOMMU dedicated to SOC-integrated graphic devices.

Commit <9452618e> ("iommu/intel: disable DMAR for g4x integrated gfx")
used this mechanism to disable the graphic-dedicated IOMMU for some
problematic devices. Later, more problematic graphic devices were added
to the list by commit <1f76249c> ("iommu/vt-d: Declare Broadwell igfx
dmar support snafu").

On the other hand, commit <19943b0e> ("intel-iommu: Unify hardware
and software passthrough support") uses the identity domain for graphic
devices if CONFIG_DMAR_BROKEN_GFX_WA is selected.

+       if (iommu_pass_through)
+               iommu_identity_mapping = 1;
+#ifdef CONFIG_DMAR_BROKEN_GFX_WA
+       else
+               iommu_identity_mapping = 2;
+#endif
...

static int iommu_should_identity_map(struct pci_dev *pdev, int startup)
{
+        if (iommu_identity_mapping == 2)
+                return IS_GFX_DEVICE(pdev);
...

In the following driver evolution, CONFIG_DMAR_BROKEN_GFX_WA and
quirk_iommu_igfx() are mixed together, causing confusion in the driver's
device_def_domain_type callback. On one hand, dmar_map_gfx is used to turn
off the graphic-dedicated IOMMU as a workaround for some buggy hardware;
on the other hand, for those graphic devices, IDENTITY mapping is required
for the IOMMU core.

Commit <4b8d18c0> "iommu/vt-d: Remove INTEL_IOMMU_BROKEN_GFX_WA" has
removed the CONFIG_DMAR_BROKEN_GFX_WA option, so the IDENTITY_DOMAIN
requirement for graphic devices is no longer needed. Therefore, this
requirement can be removed from device_def_domain_type() and igfx_off can
be made independent.

Fixes: 4b8d18c0

 ("iommu/vt-d: Remove INTEL_IOMMU_BROKEN_GFX_WA")
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Link: https://lore.kernel.org/r/20240428032020.214616-1-baolu.lu@linux.intel.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>