drm/i915/gt: rename CNL references in intel_engine.h

With the removal of CNL, let's consider ICL as the first platform using
that index.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210728220326.1578242-4-lucas.demarchi@intel.com