drm/i915: Skip capturing errors from internal contexts

We don't want to report errors on the internal contexts to userspace,
suppressing their own, so treat them as simulated errors. These mostly
arise inside selftests and so are simulated anyway. For the rest, we can
rely on the normal debug channels in CI.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200128113426.3711294-1-chris@chris-wilson.co.uk