drm/i915: Add function to set SSEU info per platform

Add a new function to allow each platform to set maximum
slice, subslice, and EU information to reduce code duplication.

Signed-off-by: Stuart Summers <stuart.summers@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190823160307.180813-3-stuart.summers@intel.com