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

Remove support for CNL as it's highly untested, probably broken, and
there is no real platform that requires this code. This is part of CNL
removal from i915.

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/20210728215946.1573015-18-lucas.demarchi@intel.com