drm/i915/gt: Mark the GT as dead when mmio is unreliable

After we detect that mmio is returning all 0xff, we believe that the GPU
has dropped off the pci bus and is dead. Mark the device as wedged such
that we can propagate the failure back to userspace and wait for
recovery.

Signed-off-by: Chris Wilson <chris.p.wilson@intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240807091014.469992-1-andi.shyti@linux.intel.com