drm/i915: capture aux page table error register

TGL introduced a feature in which we map the main surface to the
auxiliary surface. If we screw up the page tables, the HW has a
register to tell us which engine encounters a fault in the page table
walk.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
[ickle: Be brave and apply to gen12]
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20191025121718.18806-1-lionel.g.landwerlin@intel.com