drm/i915: Add function to determine if a slice has a subslice

Add a new function to determine whether a particular slice
has a given subslice.

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/20190823160307.180813-9-stuart.summers@intel.com