drm/i915/gt: remove explicit CNL handling from intel_sseu.c

CNL is the only platform with GRAPHICS_VER == 10. With its removal we
don't need to handle that version anymore.

Also we can now reduce the max number of slices: the call to
intel_sseu_set_info() with the highest number of slices comes from SKL
and BDW with 3 slices. Recent platforms actually increase the
number of subslices so the number of slices remain 1.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210728220326.1578242-3-lucas.demarchi@intel.com