drm/i915: Add function to set subslices

Add a new function to set a set of subslices for a given
slice.

v2: Fix typo in subslice_mask assignment

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/20190823160307.180813-7-stuart.summers@intel.com