drm/i915: Squeeze iommu status into debugfs/i915_capabilities

There's no easy way of checking whether iommu is enabled for the GPU
(you can grep dmesg if you know the device, or you can grep
i915_gpu_info if that's available). We do have a central
i915_capabilities with the intent of listing such pertinent information,
so add the iommu status.

Suggested-by: Martin Peres <martin.peres@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Martin Peres <martin.peres@linux.intel.com>
Cc: Tomi Sarvela <tomi.p.sarvela@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Acked-by: Martin Peres <martin.peres@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190911114655.9254-1-chris@chris-wilson.co.uk