drm/i915: rename CNL references in intel_dram.c

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

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/20210728215946.1573015-21-lucas.demarchi@intel.com