drm/i915: remove explicit CNL handling from intel_pch.c

Remove references for CNL from pch detection.

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/20210728215946.1573015-19-lucas.demarchi@intel.com