drm/i915: Add subslice stride runtime parameter

Add a new parameter, ss_stride, to the runtime info
structure. This is used to mirror the userspace concept
of subslice stride, which is a range of subslices per slice.

This patch simply adds the definition and updates usage
in the QUERY_TOPOLOGY_INFO handler.

v2: Add GEM_BUG_ON to make sure ss_stride is valid

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/20190823160307.180813-4-stuart.summers@intel.com