drm/i915: Record default HW state in the GPU error state

It may be of interest to both compare the active HW context against the
default (aka NULL) context, to see what has been changed and if either are
corrupt.

v2: Rename the fake vma as fake.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171126220901.14735-1-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>