drm/i915: Refactor instdone loops on new subslice functions

Refactor instdone loops to use the new intel_sseu_has_subslice
function.

Signed-off-by: Stuart Summers <stuart.summers@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190823160307.180813-10-stuart.summers@intel.com