drm/i915/gt: Restore dropped 'interruptible' flag

Lost in the rebasing was Tvrtko's reminder that we need to keep an
uninterruptible wait around for the Ironlake VT-d w/a

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191006165002.30312-3-chris@chris-wilson.co.uk