drm/i915: Add new function to copy subslices for a slice

Add a new function to copy subslices for a specified slice
between intel_sseu structures for the purpose of determining
power-gate status. Note that currently ss_stride has a max
of 1.

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/20190823160307.180813-11-stuart.summers@intel.com