VT-d: don't panic/warn on iommu=no-igfx

VT-d: don't panic/warn on iommu=no-igfx

When operating on an Intel graphics device, iommu_enable_translation()
panicked (force_iommu==1) or warned (force_iommu==0) about the BIOS if
is_igd_vt_enabled_quirk() returned 0. That's good if the actual BIOS
problem has been detected. But since commit 1463411, returning 0 could
also happen if the user simply passed "iommu=no-igfx", in which case
bailing out with an info message (instead of a panic/warning) would be
more appropriate.

The panic broke the combination "iommu=force,no-igfx", and also the case
where "iommu=no-igfx" is passed but force_iommu=1 is set automatically
by x2apic_bsp_setup().

Move the iommu_igfx check from is_igd_vt_enabled_quirk() into its only
caller iommu_enable_translation(), and tweak the logic.

Signed-off-by: Rusty Bird <rustybird@openmailbox.org>
Acked-by: Kevin Tian <kevin.tian@intel.com>